ExternalCloseButton, IsPinnable->IsPinned, Docs.

Added property ExternalCloseButton
Renamed property IsPinnable to IsPinned
OverrideExternalCloseButton + OverrideIsPinned

Added property OverrideExternalCloseButton
Added property OverrideIsPinned
Flyouts IsPinned functionality
Merge pull request #1000 from 100GPing100/patch-3

Close Flyouts with external click (makes #965 obsolete)