media-keys: Rename ShellKeyBindingMode to ShellActionMode

The type was renamed in gnome-shell to account for usage besides
keybindings (namely gestures), so do the same here to keep the
headers in sync.
media-keys: Adjust to ShellKeyGrabber changes

The AcceleratorActivated signal now uses a dictionary for additional
parameters to make future additions easier.
Export the session bus ID as an xsetting

This will allow GTK+ to detect ssh forwarded display situations
where app menus won't work, and handle it better.

https://bugzilla.gnome.org/show_bug.cgi?id=741740
gnome-settings-daemon: split out OSD from org.gnome.Shell

OSD can be reused in other desktop environments that are using
GNOME components.
common: GsdShell -> GsdShellOSD
wacom: GsdShell -> GsdShellOSD
media-keys: use GsdShellOSD
media-keys: watch org.gnome.Shell.KeyGrabber dbus name
media-keys: watch org.gnome.Shell.Screencast dbus name
media-keys: use org.gnome.Shell.Screenshot as dbus name