Added a "Clean.xaml" file which auto imports every "Clean" control style. Added a CleanGroupBox style.
Adapted header stuff from the original MetroGroupBox style.
Fixed the rebase.
Tweaked the CleanWindowDemo.
Updated the CleanWindowDemo again to kind of look like @flagbug 's Espera.
Merge pull request #1117 from Amrykid/more-clean

[WIP] Clean style updates