Add theme version checking

Add a themeversion value to the ui.xml details section. Check this
value against a define during early theme loading. If we are
loading a custom theme, then reject the custom theme if the
version does not match (or not defined) and load the stock theme.
If it is the stock theme, show a warning but continue loading.

Also fix an issue with changing overlays. Not sure how it ever
worked before, but it works now.

TLDR: Use the stock theme if the custom theme version does not
match.

Change-Id: I0277cbe035cbec7e16b6327500762dadf9c1c5c8