Fix #228: Move LinkedModeHandler code, check for disabled mode.

Moved LinkedModeHandler to the net.sourceforge.eclipse.interceptor
package, made it so that LinkedModeHandler is created in
VimInputInterceptorFactory instead of the UserInterfaceService.

LinkedModeHandler now checks if the VrapperPlugin is enabled before
switching modes.
Cleanup after #228: remove unused ModeChangeHintReceiver.