Use DockPanel for Flyout

Using `DockPanel`, instead of `StackPanel`, for `Flyout`s.  This allows
content to scale on resize, it allows for forcing content to the bottom
of the `Flyout` -- such as confirm/cancel buttons.
The panorama demo can be exited by pressing the escape key.
Added keys to ListBox and ListBoxItem for based on
Merge pull request #343 from stefan-schweiger/patch-1

Added keys to ListBox and ListBoxItem
Merge pull request #340 from Amrykid/panorama-demo-exit

The panorama demo can be exited by pressing the escape key.
Make sure timer is stopped on unload so panorama can be garbage collected
Merge pull request #335 from EvilClosetMonkey/master

Use DockPanel in Flyouts instead of StackPanel.
Merge pull request #344 from mrosack/master

Allow Panorama to be Garbage Collected
Merge branch 'master' of github.com:MahApps/MahApps.Metro