Merge branch 'Penguinum/Antiwall' into 'master'

Penguinum/antiwall

Result of [this thread](http://forums.xonotic.org/showthread.php?tid=5441) and [this conversation](https://gitlab.com/xonotic/xonotic-data.pk3dir/commit/7732979dccd0be2d113894202177195626e08ddf#note_1142181).

What was added: fade-out/fade-in for entities, initialized by G_CLIENTMODEL_INIT (at the moment, they are: func_clientwall, func_clientillusionary, misc_clientmodel), ability to deactivate (alpha = 0, solid = SOLID_NOT) or activate (normal condition) them by trigger. Could be useful, for example, for [MrDetonia's map](http://forums.xonotic.org/showthread.php?tid=5251&pid=70359#pid70359).

If you want to see it in action, there is a [test map](https://drive.google.com/file/d/0B4nZJEEvT4LsLTNzazA1VFBLVFk/view?usp=sharing).

See merge request !132