Add trident map

Apparently there is a vote about adding trident as another default map for Xonotic. For legal things to be clear: I release trident map under GNU GPL 3 or any later version. I added license entry and file in last commit, so it is also clear from source files.

EDIT: the vote passed - https://xonotic.org/teamvotes/432/

maps/trident.map

@@ -2486,12 +2486,12 @@ brushDef
 {
 patchDef2
 {
-map_trident/base_concrete4b_captop
+trak4x/base-concrete4b
 ( 3 3 0 0 0 )
 (
-( ( 992 -192 128 0 0 ) ( 992 -192 160 0 -0.5 ) ( 992 -192 192 0 -1 ) )
-( ( 992 -384 128 1.5 0 ) ( 992 -384 160 1.5 -0.5 ) ( 992 -384 192 1.5 -1 ) )
-( ( 800 -384 128 3 0 ) ( 800 -384 160 3 -0.5 ) ( 800 -384 192 3 -1 ) )
+( ( 992 -192 128 0.5 0.5 ) ( 992 -192 160 0.5 0.25 ) ( 992 -192 192 0.5 0 ) )
+( ( 992 -384 128 2 0.5 ) ( 992 -384 160 2 0.25 ) ( 992 -384 192 2 0 ) )
+( ( 800 -384 128 3.5 0.5 ) ( 800 -384 160 3.5 0.25 ) ( 800 -384 192 3.5 0 ) )
 )
 }
 }