Fix crash when showing external send GUI
Fix 'Location window is blank' (#5538)

Call set_session() on the location window from ARDOUR_UI::set_session(),
and show_all() on the location window's LocationUI in set_session,
so that it displays the markers and ranges of the current session.
common spelling of infinity (lowercase as libc's printf)
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)
add lib/LV2 path akin to lib/surfaces, to allow bundling of LV2 plugins in an ardour release.  My first attempt to push...
basic window infrastructure for meterbridge
implement meterbridge & meter-strip
meter-bridge details:

* sort routes
* draw metric area
* layout & style
meterbridge: fix initial and max window size.