Lookups/GPOS/AddSubtable dialog bigger, reveal controls

Make the Lookups/GPOS/Add Subtable dialog slightly bigger to expose
some previously hidden controls.

Making the dialog taller exposes the "OK" and "Cancel" buttons that
were previously hidden down below the dialog window's bottom border.
Making the dialog a bit wider exposes the rest of the vertical scrollbar
which was only half visible (under cygwin).

Steps to reproduce are in issue #1446, "GPOS Anchor classes "Add Subtable"
dialog too short"
Merge pull request #1565 from tshinnic/lookups_gpos_addsubtable_bigger

Lookups/GPOS/AddSubtable dialog bigger, reveal controls