Changed ComboBox Selection Style to better fit ListBox, ListView, etc. style

Changed ComboBox Selection Style to better fit ListBox, ListView, etc. style

-) Foreground AccentSelectedColorBrush when selected
-) Background AccentBrush when selected
-) Background AccentColor3 when mouseover
Merge pull request #450 from stefan-schweiger/patch-3

Changed ComboBox Selection Style to better fit ListBox, ListView, etc. style