Add 3 new gcode settings

- Add bool to set if the start_gcode needs to be the first gcode run.  Requires `machine_start_gcode_first` be added to the machine UI as a checkbox
- Add a prestart_gcode to extruders, this will allow use set temperatures before the change call so we can preheat the next extruder.  Requires `machine_extruder_prestart_code` text box be added to the extruder UI page.
- Add a time duration to calculate time required to change extruders. Requires `machine_extruder_change_duration` float box to be added to the exturder UI page.

src/gcodeExport.cpp

@@ -682,6 +682,11 @@ bool GCodeExport::initializeExtruderTrains(const SliceDataStorage& storage, cons
 
     writeComment("Generated with Cura_SteamEngine " CURA_ENGINE_VERSION);
 
+    if (mesh_group_settings.get<bool>("machine_start_gcode_first"))
+    {
+        writeCode(mesh_group_settings.get<std::string>("machine_start_gcode").c_str());
+    }
+
     if (getFlavor() == EGCodeFlavor::GRIFFIN)
     {
         std::ostringstream tmp;
@@ -693,8 +698,11 @@ bool GCodeExport::initializeExtruderTrains(const SliceDataStorage& storage, cons
         processInitialLayerTemperature(storage, start_extruder_nr);
     }
 
+    if (!mesh_group_settings.get<bool>("machine_start_gcode_first"))
+    {
+        writeCode(mesh_group_settings.get<std::string>("machine_start_gcode").c_str());
+    }
     writeExtrusionMode(false); // ensure absolute extrusion mode is set before the start gcode
-    writeCode(mesh_group_settings.get<std::string>("machine_start_gcode").c_str());
 
     // in case of shared nozzle assume that the machine-start gcode reset the extruders as per machine description
     if (Application::getInstance().current_slice_->scene.settings.get<bool>("machine_extruders_share_nozzle"))
@@ -1267,6 +1275,29 @@ void GCodeExport::writeZhopEnd(Velocity speed /*= 0*/)
 
 void GCodeExport::startExtruder(const size_t new_extruder)
 {
+    const auto extruder_settings = Application::getInstance().current_slice_->scene.extruders[new_extruder].settings_;
+    const auto prestart_code = extruder_settings.get<std::string>("machine_extruder_prestart_code");
+    const auto start_code = extruder_settings.get<std::string>("machine_extruder_start_code");
+    const auto start_code_duration = extruder_settings.get<Duration>("machine_extruder_start_code_duration");
+    const auto extruder_change_duration = extruder_settings.get<Duration>("machine_extruder_change_duration");
+
+    // Be nice to be able to calculate the extruder change time verses time
+    // to heat and run this so it's run before the change call. **Future note**
+    if (! prestart_code.empty())
+    {
+        if (relative_extrusion_)
+        {
+            writeExtrusionMode(false); // ensure absolute extrusion mode is set before the prestart gcode
+        }
+
+        writeCode(prestart_code.c_str());
+
+        if (relative_extrusion_)
+        {
+            writeExtrusionMode(true); // restore relative extrusion mode
+        }
+    }
+
     extruder_attr_[new_extruder].is_used_ = true;
     if (new_extruder != current_extruder_) // wouldn't be the case on the very first extruder start if it's extruder 0
     {
@@ -1278,15 +1309,16 @@ void GCodeExport::startExtruder(const size_t new_extruder)
         {
             *output_stream_ << "T" << new_extruder << new_line_;
         }
+        // Only add time is we are actually changing extruders
+        estimate_calculator_.addTime(extruder_change_duration);
     }
 
+    estimate_calculator_.addTime(start_code_duration);
     current_extruder_ = new_extruder;
 
     assert(getCurrentExtrudedVolume() == 0.0 && "Just after an extruder switch we haven't extruded anything yet!");
     resetExtrusionValue(); // zero the E value on the new extruder, just to be sure
 
-    const auto extruder_settings = Application::getInstance().current_slice_->scene.extruders[new_extruder].settings_;
-    const auto start_code = extruder_settings.get<std::string>("machine_extruder_start_code");
     if (! start_code.empty())
     {
         if (relative_extrusion_)
@@ -1302,9 +1334,6 @@ void GCodeExport::startExtruder(const size_t new_extruder)
         }
     }
 
-    const auto start_code_duration = extruder_settings.get<Duration>("machine_extruder_start_code_duration");
-    estimate_calculator_.addTime(start_code_duration);
-
     Application::getInstance().communication_->setExtruderForSend(Application::getInstance().current_slice_->scene.extruders[new_extruder]);
     Application::getInstance().communication_->sendCurrentPosition(getPositionXY());