set visibility to hidden for invisible flayouts

set visibility for flyouts to hidden by default should improve
performance and a little bug if a flyout has left position (it's  a
short time visible if the main window has animation from right to left)
remove no longer required root measuring

i mean, we don't need this anymore, because we use ActualWidth and
ActualHeight of the root
ApplyAnimation only once before flyout shows
fix BeginTime to set visibility to hidden
test default visibility
forgot the FindChildren change for the test
Merge pull request #1049 from MahApps/punker76-minor-flyout-improvements

another minor flyout improvements