media-keys: Don't leak the key grabber proxy across plugin restarts

In particular, this fixes multiple calls to on_accelerator_activated()
after stopping and starting the plugin.

https://bugzilla.gnome.org/show_bug.cgi?id=737798
media-keys: Don't leak the screencast proxy across shell restarts

https://bugzilla.gnome.org/show_bug.cgi?id=737798
media-keys: Don't leak the idle GSource across plugin restarts

https://bugzilla.gnome.org/show_bug.cgi?id=737798
media-keys: Don't leak the shell proxy across plugin restarts

https://bugzilla.gnome.org/show_bug.cgi?id=737798
media-keys: Don't leak the logind proxy

https://bugzilla.gnome.org/show_bug.cgi?id=737798
media-keys: Call stop from finalize
a11y-settings: Call stop from finalize
clipboard: Call stop from finalize
color: Call stop from finalize
cursor: Call stop from finalize
housekeeping: Call stop from finalize
keyboard: Call stop from finalize
orientation: Call stop from finalize
power: Call stop from finalize
print-notifications: Call stop from finalize
screensaver-proxy: Call stop from finalize
wacom: Call stop from finalize
xrandr: Call stop from finalize
xsettings: Call stop from finalize