Added Toolbar global plugin
Updates to the core display and bindings classes

- improvements to interactive rotation command--now resume rotation from
  current value and direction is relative to horizontal motion of mouse
- most keyboard modes are now locking and not oneshot (press to turn on,
  press again (or hit escape) to turn off
- additional mouse button functionality in modes (see quick reference)
- some changes to default keyboard bindings (see quick reference)
- changes to auto cuts parameters always result in a new autocut being
  done (instead of having to explicity perform the autocut)--users seem
  to expect this
- autocenter preference changed from True/False to on/override/off

Reference viewer only: new global plugin "Toolbar" provides GUI buttons
for many operations that previously had only keyboard bindings