vogleditor: State/Render groups option

This addresses two items from issue #161.

> 1) Add a way to toggle between modes so that users can easily just see the draw calls without other clutter and clicking - perhaps a combo box so that we can easily add more hierarchy methods without consuming too much toolbar space?

The combo box is in development. But, if having the option of groups is considered advantageous, it seems like it would be a good idea to get the infrastructure in now since there is other work going on with vogleditor. It would be easier for future vogleditor-related commits to branch on this than merge later.

I've added an option to the settings dialog to enable/disable groups in the tree display. The default is to display the tree as previously.

> 2) Another request is to support glPushDebugGroup/glPopDebugGroup separately from the state  changes. I'd expect developers to know what their debug group names mean, but just seeing "State Changes" may not tell them anything useful

Debug markers (glPushDebugGroup/glPopDebugGroup) calls are now disengaged from State/Render groups. If a Debug marker is initiated then, if Render/State groups are enabled, any State/Render group
collection ends until the Debug marker collection terminates.

Development can continue with the other items mentioned in issue #161 without affecting the default apicall tree display.