Fix dynamic toolbar tooltip updates on wxGTK

wxToolbarTool::SetShortHelp() does not refresh the UI (i.e. the
internal native representation) in all ports.

At some point, wxOSX was affected, but since fixed:
https://trac.wxwidgets.org/ticket/12362

wxGTK appears to intentionally take this route
(https://github.com/wxWidgets/wxWidgets/blob833ae03c6a80dea21b775fa871af0f32289e0bd4/src/gtk/toolbar.cpp#L808)
and addressing it there is impractical because we have to support many
wxGTK versions.

Fixed by using wxToolbar::SetToolShortHelp() instead. Other nearby code
already uses this set of functions, so it seems only appropriate.

src/wx/main_toolbar.cpp

@@ -84,7 +84,7 @@ class WXMainToolbar : public MainToolbar
         if (on)
         {
             tool->SetLabel(_("Sync"));
-            tool->SetShortHelp(_("Synchronize the translation with Crowdin"));
+            m_tb->SetToolShortHelp(m_idUpdate, _("Synchronize the translation with Crowdin"));
             #ifdef __WXGTK3__
             SetIcon(4 , "poedit-sync-symbolic");
             #else
@@ -97,7 +97,7 @@ class WXMainToolbar : public MainToolbar
         else
         {
             tool->SetLabel(MSW_OR_OTHER(_("Update from code"), _("Update from Code")));
-            tool->SetShortHelp(_("Update from source code"));
+            m_tb->SetToolShortHelp(m_idUpdate, _("Update from source code"));
             #ifdef __WXGTK3__
             SetIcon(4 , "poedit-update-symbolic");
             #else