Several small cleanups and improvements.

TGLViewer and sub-classes:
- Improve help text for TGLViewer ("Help on GL viewer").
- Add support for menu-bar hiding in TGLSAViewer.
- Allow TGLEmbeddedViewer to be connected to a GED editor.
- Remove support for "camera home" on double click -- show GED editor
  of the viewer instead.

TGLEventHandler:
- Improve handling of mouse-button events:
  - Only allow single button activity at the same time.
  - Remove function-static variables.

TEveViewer:
- Enable by default menu-bar hiding for GL viewers.


git-svn-id: http://root.cern.ch/svn/root/trunk@31299 27541ba8-7e3a-0410-8455-c3a389f83636