Unneeded work on every paint:
- don't call set bounds if we only want to resize
- don't resize again if we are already invisible

General cleanup:
- use JFacePreferences.QUALIFIER_COLOR instead of hard coded string
- removed second listener for dispose events
- reduced spacing (see
https://github.com/eclipse-platform/eclipse.platform.ui/issues/702)

See https://github.com/eclipse-platform/eclipse.platform.ui/issues/708