store min/max button visibility and use it on resize mode change
really fix min/max button visibility

- add ResizeModeMinMaxButtonVisibilityConverter to handle changes from
the min/max boolean properties and the window resize mode
- add some tests
the dispatcher wants the access
Merge pull request #1376 from MahApps/1362-punker76-ShowMinMaxButton-bug

[WIP] fix not working ShowMinButton/ShowMaxRestoreButton properties