Add moveable pivot to selection widget and clips

[StickmanRed]

Features

• Drag pivot handle to reposition
• When a single clip is selected, clip saves pivot offset
• Pivot handle locks when the selection box is too small (when both width and height are smaller than 4x the pivot size)
• Alt to rotate from opposite corner

Also the selection code was significantly cleaned up. I had previously contained all the changes to the selection tool in a this.mod object... no more of that. Some of the logic was simplified as well.