Allow external translation files

Quassel will now check its data dirs (usually $prefix/share/apps/quassel) for
a directory named "translations". If this is found, translation files will be looked
for in this directory *only*. This means, that Quassel won't look in :/i18n then.

Keeping .qm files in $prefix/share/apps/quassel/translations allows for providing language
packs easily.

Also, we move loadTranslation() from util.cpp to Quassel::loadTranslation().