local monitors: make more idiomatic use of GObject

Stop abusing constructor() to do startup work, adding _start() calls
instead.

The backends themselves still use constructor() although a patch will be
following to also fix inotify.

The reason for using a separate start() call instead of constructed()
will become apparent in future commits.

https://bugzilla.gnome.org/show_bug.cgi?id=704887
inotify: stop using constructor()

and start using the new start() vcall on the local monitor classes.

I only port inotify because I am uncomfortable making changes to the
other monitor backends without having a way of testing them.

https://bugzilla.gnome.org/show_bug.cgi?id=704887
GFileMonitor: support specifying a context

Add a new "context" construct-only property to allow explicitly
specifying the context in which events should be dispatched.

https://bugzilla.gnome.org/show_bug.cgi?id=704887
glocalfile: add private worker monitor APIs

Add a convenient and race-free method of watching local files from the
GLib worker thread.

Without this, the race-free way to create a monitor that dispatches
events to the worker thread looked something like this:

 - dispatch an idle to the worker thread
 - from the idle, create the monitor and connect signals
 - from the original thread, wait (on a cond?) until the worker thread
   has finished setting up the monitor
 - read the file that you were monitoring

which is just ridiculously complicated...

To use the new API:

  monitor = g_local_file_monitor_new_in_worker ("/path/to/some/file",
                                                G_FILE_MONITOR_NONE,
                                                &error);
  g_assert_no_error (error);

  g_signal_connect (monitor, "changed", G_CALLBACK (callback), NULL);

  g_local_file_monitor_start (monitor);

'callback' will run from the GLib worker thread.

This is the reason that the start() call was introduced in the previous
commit.  The backends that don't use the start() call will have a very
thin race between creating the monitor and connecting the signal, but
hopefully they will be fixed soon.

These new APIs will be used (at least) from gdesktopappinfo to watch for
changes in the desktop file directories.

https://bugzilla.gnome.org/show_bug.cgi?id=704887