[WIP] extend feature windowcommands over flyouts #708

this pr extend the feature to show window commands over flyouts.

- [x] i split the ```WindowCommands``` into ```WindowCommands```and ```WindowButtonCommands```. the ```WindowButtonCommands``` only holds the min, max, close buttons. and the ```WindowCommands``` the additional buttons with commands.
- [ ] introduce new boolean property ```WindowCommandsOnTop``` for ```MetroWindow``` to disable the feature for the additional buttons. the min, max, close buttons are always on top!

Closes #708