More grammatical number support, esp. for monsters

This PR adds proper plural support for monster names.
This adds a new field for monster types: name_plural. This field is for the English plural of the monster name. This works very similar as in the pluralization of item names.
The monster type struct has a new virtual function, nname(unsigned int quantity) to fetch the pluralized monster name.
The name() function of the monster struct was rewritten. It has now an optional unsigned int parameter to also fetch the monster name with respect to grammatical number. If the parameter is omitted, singular (1) form is assumed, which is pretty much the same behaviour the function had before.

With this PR, the pluralized monster names are used in these areas:
* The message about the player not feeling guilty anymore after killing 100 zombie children (not zombie childs! ;)) (or similar monsters)
* The kill stats in the memorial file
* Titles of special waves in defense mode

Admittedly, there are currently not many areas of C:DDA where the plurals are actually used (and needed). But it may come in handy in the future.

Moreover, some forgotten strings which were not pluralized before are now (hopefully) pluralized. See the commits for details.

The updated string extraction scripts have been tested successfully, but a new .pot file is not included.