Add selection and control of multiple visual features

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.