Define the flyout brushes in the theme dictionary

This effectively removes the Theme class and we are more flexible when
it comes to flyout theming
Define the two brushes for the foreground and background properties and remove the rest

The inverse theme now is only applicable if the host window's theme is
either BaseDark or BaseLight, this way we resolve the theme issue with
custom themes
When looking for the inverse window theme, rely on a naming convention

If one uses the inverse flyout theme, the window theme must be affixed
with "Dark" or "Light", and both must be available
Add some words
We can't really test that anymore
Fix unit test build
Add some tests
Merge pull request #1223 from MahApps/remove-theme-property

Define the flyout brushes in the theme dictionary