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.
Snippet library: added an optional id to snippets.

Uses that id instead of the original text in the profession item list.
Merge remote-tracking branch 'dda/pr/9102' into dda-master

* dda/pr/9102:
  Snippet library: added an optional id to snippets.
  Allow professions to specific the description of the starting items.
  Professions return the combined item list including gender specific and gender-neutral items.
  Add translation for snippet text inside of item definitions:
  Allow random descriptions to be specified in the item definition:
  Allow random descriptions for all item types.
Make some snippet_library methods private.
Add a t-shirt with text using the new generalized snippets system.
Give the computer hacker profession an appropriate t-shirt.