Tags now translatable

Use the techtree translation key: `TagName_<actual name of tag>`.
Eg, `TagName_warrior` for units that have `<tag name="warrior" />`.
As with the rest of techtrees, the game setting "Allow translated
techtrees" must be enabled (otherwise reasonable defaults are shown).

Also, the TagDesc is now translated for upgrades, too. This makes
everything related to tags translatable.
Fixed warnings previously introduced into code