drag n drop improvements

[mmckegg]

• orderable should drag a placeholder and show insert mark - don't perform sort until mouse release
• this should also be applied to moving chunks around on controllers
• find all uses of window.currentDrag and try to handle using native data
• ensure that stuff can be dragged between loop drop instances (no more horrible hacks)
• allow drop of sources/processors directly on to collections
• allow file drop directly on to certain collections

[mmckegg]

Tried using placeholder sorting, but it's kinda glitchy and hard. Have improved Orderable greatly by reducing flicker/jitter when hovering larger items.