src/main: delay startup of applications until event loop is ready

src/main: delay startup of applications until event loop is ready

This mostly allows for using scripts that trigger a labwc SIGHUP
to work as expected. Before this patch, there was a race between
the event loop starting up and the autostart scripts executing.

One example of such a use-case is dynamically setting the keyboard
layout or cursor theme via environment variable based on feedback
from a dbus service like discussed in #1588.