If floating the content and a FloatPane exists,

no need to set the Pane, it's already ready to go.
Fix potential memory leak in FocusManager

If ResumeFocusTracking was called without any call to SuspendFocusTracking,
then the FocusManagerImpl would be subscribed to the windows hook event twice,
and remain subscribed even if it was disposed.
Merge pull request #239 from roken/master

#237 Fix Show(DockPanel, Rectangle) to float content at the rectangle.
Merge pull request #243 from jdh28/focusmanager-leak

Fix potential memory leak in FocusManager