Added 'magnetism' when moving sources around. Slight stickiness when centers and edges are aligned, with visual indicator of which alignment is occurring. For easier centering.
Resizing sources only works on the corners now.
"Selected source" outlines only show when source is moused over, or clicked on. Cursor changes for 'draggable' and 'resizable'.
Added a bit of leeway to source borders when checking mouse events.