Make CD track details visible whenever CD range is ticked.

Make the CD track details visible whenever the CD range box has been
ticked, rather than only when the user toggles the box on.
fix alignment of meters on midi-tracks
adjust mixer meter metric scale
fix race/endless loop on exit:

if gtk is going away while ardour updates the splash-screen:
Gtk-CRITICAL **: IA__gtk_main_quit: assertion `main_loops != NULL' failed
reset meters when changing metering-point or ports
fix metering when monitoring track input. (ran twice)
possible fix for crash on exit - err on the side of caution
fix/workaround window-proxy session-management

The window proxy defers construction of classes.
set_session() is called before the instances are available.
the proxy-manager only calls set_session() for SessionHandlePtr
but not the class-specific method..

fixes http://tracker.ardour.org/view.php?id=5566
fix save/restore [plugin] window pos&size (without save)
make window-manager session-aware
revert 3c6ff2f0 now that WM-proxy is session-aware..

also includes a few additional tweaks on top of the revert:
don't set sessions to windows managed by the WM::Manager,
and also call WM::Manager's set_session (s); when closing session.
window delete/hide is managed by WMProxy (amend to cb6f16ac)
Merge branch 'master' into cairocanvas