Log Message:
games: Add dhewm3.

dhewm 3 is a Doom 3 GPL source port. The goal of dhewm 3 is bring DOOM 3
with the help of SDL to all suitable platforms.

Compared to the original version of Doom 3, dhewm3 has many bugfixes, supports
EAX-like sound effects on all operating systems and hardware (via OpenAL
Softs EFX support), has much better support for widescreen resolutions and
has 64-bit support.

Note that while the Doom 3 source code has been released under GPL, you
still need to legally own the game and provide dhewm3 the game data to play.