Poke wmgenmenu some more

- change some variable names to better reflect their purpose, do a little
  write-up how a menu is built
- auto-detect what terminal to use for apps in need of a terminal
- fix the chunk that adds the terminal-based apps

util/wmgenmenu.h

@@ -2,8 +2,7 @@
 
 /*
  * If the program should run from inside a terminal it has
- * to finish with a space followed by '!', e.g.
- * "mutt !"
+ * to end with a space followed by '!', e.g.  "mutt !"
  */
 
 char *terminals[MAX_NR_APPS] = {