GZDoom used to have hardcoded MessageBox menu selector, 0x0d character
of console font, while the other menu is displayed is SmallFont.
It looked too ugly if SmallFont and ConFont heights are different,
and also there was no method of modifying the selector.
Now, the selector is 0x0d from SmallFont, if SmallFont contains this glyph
(its height is greater than zero), otherwise it falls back to previous
behavior (using ConFont as a source of this glyph).
To define custom MessageBox menu selector, just define 0x0d glyph for
SmallFont, and it will be displayed in the menu.
The gap between selector and menu options text is 3 pixels (as before), and
if you wish to enlarge this gap, just add some transparent columns at the
right side of 0x0d glyph.
