prepare meter type unification
share meter-type among meter instances.
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.
use digital max-peak with RMS meter
ignore state-changes during [meter] context-menu popup (gtkmm bug?)
Merge branch 'master' into meterbridge