in order to prevent glib-threading segfaults (mga#10289), just block the
CHLD signal during the window where glib create threads behind our back
(RT-120951)

previous fixes for mga#12280 resulted in Colin reporting mgaapplet
exiting (not segfaulting):

"When updates a bubble pops up:
- If you click on the background of this bubble, mgaaplet exits
- If you click on the "Install Updates" button, it pops up the password
  dialog.  Without touching anything, about 1s later the dialog seems to
  be replaced with another that says "Sorry, that didn't work, please
  try again". When this dialog appears, mgaapplet exits."

So let's clean the pile of fixes and:
- have only _one_ gtk+ main loop
- creating _one_ hidden notification before entering the main loop
- never destroy it, but use >close + >clear_actions() then ->update()
  prior calling >show() when one need to show a notification again

The old method worked on older gnome-shells, this new method works
on newer gnome-shells (see bgo#706783)

Ideally neither hacks should be necessary and maybe (eventually) we can
remove them, but for now...

Since commit 95b9cd06f14a9817090584d72830df870c591acc, we run a gtk
main loop after displaying a notification, else actions when clickong
notification buttons are ignored by gtk+/libnotify

However, if the notification is not manually closed, we never exit
this main loop.

In that case, gtk+ fails with:
(mgaapplet:9060): Gtk-CRITICAL **: gtk_window_set_accept_focus: assertion 'GTK_IS_WINDOW (window)' failed

from:
    data=<optimized out>, destroy=0x0, button=3, activate_time=5407876) at gtkmenu.c:1613

And X11 is stuck.

As a workaround, since libnotify offers no way to be notified when
notification is automatically closed, just add a timeout for exiting
the main loop.

At worse, X11 will be stuch only 5 seconds.