graphics: Generate text graphics programatically.

Remove the big text graphic images used for the menus and the
intermission screen. Instead, generate these graphics programatically
from a script. This allows them to be easily changed and extended
as desired.
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.
graphics: Add 'make clean' target for textgen.
graphics: Add missing font characters.

Add font characters for '/' and '?', needed by some text strings.
Adjust the vertical height of the ':' character, and remove some
leftover menu graphics that are no longer needed.
graphics: Make textgen script work with Python 3.

Also add copyright notices to Python files.