Visibility support plumbed through

The settings aren't hooked up to the visibility yet but the
infrastructure is there now
Hook the setting up

Added the vsvim_useeditorcommandmargin setting.  This now controls
whether or not status updates go to the command margin or the status
bar.
Hooked up setting UI
Reduced the timer

The feature felt a bit laggy at the previous setting.  Updated the
polling interval to be shorter and changed it to normal priority.
Fixed up the unit tests

The only remaining failure was the memory leak tests.  It turns out the
test code was force creating CommandMargin which is an
IWpfTextViewMargin.  The WPF editor code path never asked for the
creation of the margin hence it never bothered to run Dispose and this
leads to a memory leak.