plugins: screensaver-proxy: Fix a segfault when uninhibiting

When an application requests that the screensaver be inhibited a cookie
is created for it and put into a hash table. The function
(csd-screensaver-proxy-manager.c::handle_method_call) that does this
also handles the uninhibiting case and would remove the cookie from the
hash table.

After this the name_vanished_cb callback function is called, it looks
through the cookie hash table for the relevant cookie and gets the
associated sender value. Unfortunately due to the above removal of the
cookie entry this never finds anything and then when it tries to use the
still uninitialised sender variable to remove an entry from the watch
hash table, you get a segfault.

Seeing as the above loop (in name_vanished_cb) removes the cookie entry
itself we can simply remove the call to g_hash_table_remove in
handle_method_call that removes the cookie entry.

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>
plugins: screensaver-proxy: Allow /ScreenSaver interface path

This is essentially a port of Bastien Nocera's patch to
g-s-d/screensaver-proxy[0].

The problem was that under cinnamon 2.x, the current version of the
vlc[1] media player is using the /ScreenSaver path to
org.freedesktop.ScreenSaver which c-s-d screensaver-proxy didn't
recognise

    method call sender=:1.118 -> dest=org.freedesktop.ScreenSaver
     serial=3 path=/ScreenSaver; interface=org.freedesktop.ScreenSaver;
     member=Inhibit
       string "vlc"
       string "Playing some media."
    error sender=:1.24 -> dest=:1.118
     error_name=org.freedesktop.DBus.Error.UnknownMethod reply_serial=3
       string "No such interface 'org.freedesktop.ScreenSaver' on object
       at path /ScreenSaver"

and the screensaver would kick in after a time while watching a video.

This adds the definition for the /ScreenSaver freedesktop.org interface
path and allows vlc to properly inhibit the screensaver.

[0] -
https://git.gnome.org/browse/gnome-settings-daemon/commit/?h=gnome-3-10&id=fd0e2b107cdf5e82a034fc9a30753259ecc3d267
[1] -
http://git.videolan.org/?p=vlc.git;a=commitdiff;h=5fe4b004e254b6091f997b394190ad1609c67aa8

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>
Merge pull request #30 from ac000/screensaver-proxy

Screensaver proxy