debug-window: prevent proxy_invalidated_cb to be called after dispose

https://bugzilla.gnome.org/show_bug.cgi?id=692793
debug-window: prevent using the store when disposing

Using g_signal_connect_object() isn't enought as it prevents from calling the
callback when the windows has been destroyed but not while disposing it.

https://bugzilla.gnome.org/show_bug.cgi?id=692793
debug-window: fix proxy leak

gtk_list_store_get() gives us a new ref.

https://bugzilla.gnome.org/show_bug.cgi?id=692793
debug-window: fix leak on all TpDebugClient

The store keeps its own ref and the client is reffed while the async call so
we can unref it right away.

https://bugzilla.gnome.org/show_bug.cgi?id=692793
debug-window: disable Debug on all proxies when exiting

https://bugzilla.gnome.org/show_bug.cgi?id=692793