Allow random descriptions for all item types.

Moves the snippet_category into the itype class and removes the it_stationary class as it is not needed anymore.
Random description are used when the item type has a snippet category.
Allow random descriptions to be specified in the item definition:

Instead of having separate entries, the snippet category is automatically created and filled with the given snippets.

Also adds check for snippet category to exist.
Add translation for snippet text inside of item definitions:
Professions return the combined item list including gender specific and gender-neutral items.
Allow professions to specific the description of the starting items.
Changes skill in weldrig recipe to use mechanics.

Most of the books that contain it are mechanics or electronics books,
and the skill it uses is mechanics, not fabrication.
Made pillows require both components.
Make sure pointers are initialized.
tabkiller 4: mall
Also fixed down blanket.
Stupid keys being nearby. Fixed typo.
More helpful debug/too-much-ammo-spawned message.

I've not actually seen this debug message happen, but assuming it ever
does, it'd be nice if it told you how much ammo was too much, instead of
giving you the count after clamping.
Snippet library: added an optional id to snippets.

Uses that id instead of the original text in the profession item list.
Add some flags modifying collisions to various furniture.
Misc whitespace and include tweaks.
Smash items when a vehicle wheel runs over them.
Use a bitmask instead of string flag for a wheel check.
Avoid smashing furniture if it has nocollide flag, and avoid smashing tiny furniture unless it's hit by a wheel.
Made forge transparent
Alters zombified dogs' speed ratings.

After hearing a lot of complaints about the various zombie dogs I
decided to look into their numbers. Everything looked fine except for
their
speeds, which were pretty crazy values.

Keep in mind that a normal, living dog has a speed of 150 and a normal
zombie dog has a speed of 105.

Stuff I changed:

skeleton dog had speed 160 which I reduced to 125
Z-9 and rot-weiler had speed 150 which I reduced to 105