(svn r7616) -Cleanup:
-(re)set _rename_[id/what] to -1 to catch invalid calls (main_gui.c)
-Only invalidate widget of pause/ff instead of whole window (main_gui.c)
-Remove numbering from WE_ and WC_ as it's not needed, also remove
non-existing windowclasses (window.h, openttd.h)
-Give names to some of the enums (window.h)
-In UninitWindowSystem not only free malloc'd widgets, but also reset the z-array (window.c)
-Some coding style, comments, etc.

/* $Id$ */

KNOWN BUGS / PROBLEMS:

bridges:
- Clearing tiles which may not have a bridge above can cause random bridge pieces to appear, which leads to crash in the long run

electrified rails:
Normal and elrail depots look the same. Use 'X' (transparent buildings)
   to distinguish between them
Missing curors / icons for construction (currently using the conventional ones)