Add the ability to move a selection when dragging it from inside

• Add is_inside_of_sel() that exclude the margin used for detecting the edges of the selection
• Add com.originX/Y to save the original location of the selection, while com.startX/Y is storing the original location of the cursor
• com.drawing && com.freezeX && com.freezeY all true means we don't resize, we move

Cleanups:

• Refactor selection com.startX/Y & com.originX/Y by using struct point
• Cleanup: remove unused is_shift_on in selection (was always false)
• Cleanup: remove a "fit" local pointer in on_drawingarea_motion_notify_event() to directly use gfit like in the rest of the code (it was not even used everywhere in the function declaring it)