Fix lockup when a scroll event is received outside the drawing area

This can occur if the user is dragging a component in the drawing area (hence
a pointer grab is in operation), and during this drag, the user moves the mouse
outside the drawing area and touches the scroll wheel.

We then hit the scroll event handler, mouse action handler, action dispatcher
etc.. and find we do not have a current location known. (The cursor is outside
the drawing area).  We then prompt for a location, and using a recursive main-
loop, wait for some kind (e.g. a mouse click) which gives one.

The problem appears to be caused by the fact it is likely we get a second
scroll event with the first, and that we process this from within the recursive
main-loop. We are still outside the drawing area, so prompt the user for a
location, and enter another level of recursion. (In my tests, I was able to hit
6 or 7 levels deep of main-loop recursion).

Our code fails to exit its recursive main-loop if it rucurses, and in any case,
we would not want to prompt the user "n" times in a row. The failure to exit
from recursion is probably due to the event handlers attached by the first
recursive main-loop being called prior to those attached later. (We attempt to
quit the first recursive main-loop, but cannot, as there are others now running).

As a quick work-around to the immediate problem, prevent scroll events being
received from within the recursive main-loop. This prevents the lockup, but
ideally we should re-think how we handle these events to avoid requiring the
user to click on a coordinate.

I have also added a GDK_THREADS_{LEAVE,ENTER} pair around running the main-loop,
as this is what GTK's code does for gtk_dialog_run().