This is a refactoring that is difficult to split in several pieces.

This patch implements the following changes:
- Use loops instead of serie of if/elses.
- Use enums and static arrays to define a static serie of tag types
and properties.
- Use enums for ordering and retrieval of properties of tags instead of
searching.
- Merge common code.
- Extract common code in functions.
- Mark for translation some strings.
- Plug some leaks.
- Fix some free-after-free.
- Fix issue with condition for unavailable metadata, that was
exacerbated with this refactoring.

https://bugzilla.gnome.org/show_bug.cgi?id=770586