More MVVM related changes

Add SelectedFilterIndex to Gallery and InRibbonGallery.
DropDownButton: MIssing bindings for UsesItemContainerTemplate and
ItemContainerTemplateSelector for creating quickaccessmenuitem
COmboBox: Missing binding for ItemTemplateSelector in creating
quickAccesItem. remove ActualWidth binding that causes the combobox to
not be visible as quickaccessitem
RibbongroupBox: Fix DialogLauncherButtonKeyTipKeys string. The correct
one is LauncherKeys. Also added missing
RibbonControl.BindQuickAccessItem in creating quickaccesitem for
groupbox.
Updated ItemsSourceHelper (common code for handling ItemsSource changes)