Make all visual tools support selecting and manipulating multiple visual
features at once, allowing multiple lines to be moved at once, entire
vector clips to be translated, etc. Controls:
- Left click: Select control clicked control only
- Ctrl-left click: Add/remove control to selection
- Drag control (with or without ctrl): Move all selected controls,
after setting/adding to selection if target is not in the selection
- Click on no control: Clear selection
Lots of little stuff to fix still.
Updates #513.
Originally committed to SVN as r4322.
23972b10bc