local monitors: make more idiomatic use of GObject

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