[1.8] first connect to settings, then read them.

fixes the issue with GLib >= 2.43,
https://git.gnome.org/browse/glib/commit/?id=8ff5668a458344da22d30491e3ce726d861b3619

When switching monitors AND displaying notifications at the
same time, a rare situation might occur when notification
window will be added to two or more stacks. One of the
stacks will be removed soon enough, but the "destroy"
handler will persist and will be triggered when the window
disappears. This results in a reference to a freed memory
and (usually) a daemon crash, a typical one is
https://retrace.fedoraproject.org/faf/reports/359836/ for
example.

So, the removal handlers referring to a deleted stack must
be cancelled prior to removal.

Closes https://github.com/mate-desktop/mate-notification-daemon/pull/34

Closes https://github.com/mate-desktop/mate-notification-daemon/pull/38

Closes https://github.com/mate-desktop/mate-notification-daemon/issues/37

Some window manager such as i3-wm may cause
wnck_screen_get_active_workspace to return NULL.
Add a sanity check before using the return value.