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
UseAnimations disables overlay animations.
Fixed a margin issue in InputDialog.
Fixed the foreground for accented InputDialogs.
Merge pull request #1050 from Amrykid/dialog-patch1

MetroDialogs Patch #1
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
Fixes #1053
Good catch by @lycilph
FlipView was using Height instead of ActualHeight.
Merge branch 'qd-rangeslider-squash'
add attached property UseSpellCheckContextMenu
Fix #1051
Fix #1052
finished changes

- rename property to IsSpellCheckContextMenuEnabled
- don't show separator if suggestions are empty
- raise InvalidOperationException if element isn't a TextBoxBase
- add property to TextBox and RichTextBox at main window example
use opacity mask to avoid warnings
Merge pull request #1061 from MahApps/955-punker76-flyout-binding-warnings

fix flyout warnings
Merge pull request #1056 from MahApps/1048-punker76-textbox-spellchecking-menu

enable spell checking context menu