First tests towards internationalization with gettext.
Load translations for the standard Qt components.
Fixes to the translation system.

- Fallback to null translator when no translations file is found.

- Using native Qt file dialogs instead of system dialogs, because
native dialogs did not get translated in KDE, despite they being in
other KDE applications.
Add Galician translation and fix details in the Spanish one.

This is the first completely-translated version of the program.
Merge branch 'i18n' into development