[WIP] Fixes in translation generation (itstool)

[nelson6e65]

This will fix issues on including unnecessary strings to be translated, like in monsters/*.xml (#299)

• 1️⃣ Ignore strings that should not be translated from XML files (_monsters_), by adding ITS rules. Only "name" attribute/element are collected for translation.
  Rules for Monsters are in monsters/_monsters.its file, and can be used by running:

$ itstool monsters/monster*.xml -o "monsters.pot" -i "monsters/_monsters.its"

• 2️⃣❓ Insert ITS rules directly in *.xml files to do not use -i command line option. Then, translatable strings can be collected just by runnig:

$ itstool monsters/monster*.xml -o "monsters.pot"

Feedbacks

• Is it really needed 2️⃣ or 1️⃣ is enough? (Check nelson6e65@f386992).
  I think 2️⃣ is unnecessary, and with only 1️⃣ the *.xmls kept simple and untouched. But I don't know how @4144 collect the strings exactly.