I've found Unix Pageant's GTK password prompt to be a bit flaky on
Ubuntu 18.04. Part of the reason for that seems to be (I _think_) that
GTK has changed its internal order of setting things up, so that you
can no longer call gtk_widget_show_now() and expect that when it
returns everything is ready to do a gdk_seat_grab. Another part is
that - completely mysteriously as far as I can see - a _failed_
gdk_seat_grab(GDK_SEAT_CAPABILITY_KEYBOARD) has the side effect of
calling gdk_window_hide on the window you gave it!
So I've done a considerable restructuring that means we no longer
attempt to do the keyboard grab synchronously in gtk_askpass_setup.
Instead, we make keyboard grab attempts during the run of gtk_main,
scheduling each one on a timer if the previous attempt fails.
This means I need a visual indication of 'not ready for you to type
anything yet', which I've arranged by filling in the three drawing
areas to mid-grey. At the point when the keyboard grab completes and
the window becomes receptive to input, they turn into the usual one
black and two white.
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