Posibility to group the TileViewItems needed

Add TileViewItem.HeaderBackground property.

DECLINED: There is SDK sample available demonstrating how you can change the background color of the header of each item.
https://github.com/telerik/xaml-sdk/tree/master/TileView/ChangeHeadersBackground

If a RadTileViewItem hosts several controls (e.g. Buttons, RadioButtons) users should be able to change the focused element using arrow keys.