Update German translations and (slightly) expand localisation

I've updated many of the German entries in the stringtable.xml file to provide a better translation or fix some mistakes.

While already working on translation, I've also included localisation for a few "hardcoded" strings found in fn_useItem.sqf and fn_setupActions.sqf (since I don't speak French, Italian or Portuguese, I've simply copy-pasted the English translation into those entries).
Entries from fn_useItem.sqf have been added to the *More_Item_Strings* / *STR_ISTR_\** category, the translations from fn_setupActions.sqf have been added to a new category called *Player_Actions* / *STR_pAct_\** since I didn't find a fitting place for them.