[BUG] When sharing a drawing it always gets the same name - simple-draw.png

[rogercreagh]

When sharing a drawing it always gets the same name - simple-draw.png

This happens even if you have saved the file and given it a name and causes problems with the receiving app as a new file shared may overwrite a previous one.

Suggest a default filename of simple-draw-YYYY-MM-DD_HHMMSS.png including the date and time if the sketch has not been saved, and using the filename if it has already been saved locally.

It is sometimes convenient to make a quick sketch to share into another app without bothering to save it from within simple-draw, so a human-readable more likely unique name using datetime would be a good solution without having to prompt for a name.