Combine "Bonus" and " XP: " in MenuCharacter

The "XP" part wasn't being translated (I thought it was), so we might as
well create a single string here.
Forgot a parenthesis
Don't apply bonus_per_mental twice on healing and shielding
Update engine.pl.po

Written missing translation for "Trade"
Merge pull request #502 from Pawonashi/patch-3

Update engine.pl.po
Merge pull request #497 from dorkster/bugfixes

Allow randomized quantities for map loot
Merge pull request #498 from dorkster/translate

Combine "Bonus" and " XP: " in MenuCharacter