PSThar - Selected tile cmdlets

Today I implemented a simple pair of cmdlets for PSThar: Get-SelectedTile and Set-SelectedTile. Each takes either a plane number (0 to 2) or one of three switches (e.g. -Background). The setter version, of course, takes a new tile ID to place in the appropriate selected tile bay. It also performs the bookkeeping task of sending a "selected tile changed" notification.