UI localization.

The engine will now load .po files in the following order:

l10n/LANG.po ("common" domain)
ui/l10n/LANG.po ("UI" domain)
ui/uitheme/l10n/LANG.po ("UI" domain)

Theme .po files take precedence over global UI .po files found in ui/l10n. As the last resort, the localization subsystem will search for the string in the "common" domain (ui/l10n/LANG.po).