dehacked: Add intermission text screens.

This replaces the default text screens with text screens more
relevant to Freedoom's level progression. The Double Impact end
text screen is imported in a modified form, and the episode 1-3
text screens ought to be considered temporary.
dehacked: Add level names.

These are some initial level names based on discussion on the Doomworld
Freedoom forum thread found here:
http://www.doomworld.com/vb/freedoom/66592-lets-name-freedooms-levels/
text: Add freedoom.bex as build dependency.

The text must be rebuilt if the contents of the Dehacked file
change, as level names are imported from it.
dehacked: Add name for E1M6.

This gives us a full set of level names for episode 1.
lumps: Record new Boom 2.02 demos for Doom 1 and 2.