Added an 'AccentedSquareButton' style and updated every accent to have an 'IdealForegroundColor' key. Updated Dialogs to use that so that the accented button would have a hover-over effect.

Signed-off-by: Amrykid <amrykid@gmail.com>
Fixed AccentedSquareButton's IsMouseOver Foreground.
Adjusted Yellow's IdealForegroundBrush.
Accented Dialogs use IdealForegroundBrush.
Updated the demo to allow for Accented Dialogs.
Merge pull request #845 from Amrykid/accented-squarebtn-and-dialogs

AccentedSquareButton, IdealForegroundColor key and hover-effect for Dialog buttons.