Reduced the .NET framework requirement from 4.5.1 to 4.5.
Added support for multiple levels in menus via GroupItem actions.
Reworked API:
 - only one [ExportCLASS] attribute is now required to export something
 - Fallback Tab metadata is not longer specified in the Tab but it is itself something constant in TabService
Improved fallback tab by providing a default tab with a default view
 - Prevents crashing of the program if no "default_tab" is specified
 - Does not allow plugin developers to redefine a fallback tab
Merge branch 'feature/api-rework' into develop
Fixing empty tooltip in Actiongroups
Hiding PanelsView when there are no items.
Merge branch 'develop' into feature/multiple-panels

Conflicts:
	SuperbEdit.FolderPanel/ViewModels/FolderPanelViewModel.cs
Hiding parent ContentControl when panels have no items.
Added panels submenu