introduce MetroTheme and some other changes

- add MetroTheme class (works like Accent class)
- add DefaultMetroThemes property to ThemeManager (so users can add
there own themes)
- overload some methods to work with new and old one
rename MetroTheme -> AppTheme
change obsolete text warning
put all obsoletes in one region
remove reflection invalidation
DebuggerDisplay
some more helper methods
DefaultAccents->Accents, DefaultAppThemes->AppThemes
AddAccent, AddAppTheme
Merge pull request #1205 from MahApps/punker76-thememanager-metrotheme

ThemeManager improvements without braking change