Initial revisions on checking if avalondock is a good solution for implementing multiple panels
Fixing active item not being updated and added panels.
Fixing Panels.
Implemented back TryClose.
Implemented back has changes in the item header.
Added stubs for loading and saving layout data.
Continued the stub for saving and restoring panel positions.
Added readonly visual indicator in tab.
Improvements in laying out panels
 - Implemented panel default position loading.
 - Removed some classes which are not needed anymore.
Added possibility to set panel default position by code
Merge branch 'feature/avalondock' into develop

Conflicts:
	SuperbEdit/AppBootstrapper.cs
	SuperbEdit/ViewModels/ShellViewModel.cs
Fixing layout saving panels.