Controls:Tile can't tell if enabled or not

[bloritsch]

I have some tiles bound to a command, and sometimes the command can't be executed (for example, can't continue to edit a project if the user never started one to begin with). All tiles look like they can be clicked until the user attempts to do so. I would expect a disabled tile to be grayed out, or have a semitransparent layer over top to make it look unclickable.

(Release version, Mahapps.Metro 1.1.2)