Fix window resizing in GTK 3 PuTTY.

Fix window resizing in GTK 3 PuTTY.

In GTK 3, it was impossible to resize the window smaller than it
started off, because the size request on the drawing area was taken as
a minimum. (I can't actually remember how the GTK 2 version doesn't
have this problem too.)

Fixed by instead setting the initial window size using
gtk_window_set_default_geometry() (having first set up the geometry
hints to reflect the character cell size).