Big sprawling dialog-box commit covering all sorts of things.

Buttons now have an `iscancel' flag to go with `isdefault';
dlg_last_focused() now explicitly passes the control it _doesn't_
care about (`I want the last control that had focus and isn't this
one'); and in the GTK implementation, various fixes have happened,
notably including arrow keys working sensibly in list boxes and the
treeview and short font aliases being expanded correctly to
initialise the font selectors.

[originally from svn r2958]