graphics: Adjust offsets of menu text.

Some of the text graphics on the options and sound menus were
misaligned using the new generated graphics. Adjust these so that
they are spaced out more evenly.
graphics: Add overrides for source port text lumps.

Some source ports (PrBoom, Eternity Engine, etc.) come with their own
PWAD files that include custom text graphic lumps for extended menus.
Include some overrides in Freedoom for common ones, so that when
playing in these ports the menus have a consistent look and feel.