sketch, new activity

The activity is ready for review.

A few minor things left todo:

• Add the condition to disable the activity if software rendering is used.
• Fix packaging for QtQuick Shapes and qml/QtCore.
• on Windows, GCreationHandler fails to save image in the activity folder (but undo/redo snapshots are properly saved in the temp folder, so probably not a big thing to fix)... Need to check if it fails also for other activities json files, or if it's specific to images.
• improve GCreationHandler visually (there are general and specific-to-images little things to fix... Though that can be done after the merge of this activity, not directly related...)

^ all done.