Combobox for language and themes

OK, I have implement the corresponding combobox for language and theme selection.

First... **use it**... to know if it is **enough** intuitive... then read the brief explanation below:

The implementation is based in selection/options combobox, with some tricks to deal with the automatic language detection and the manual one. As a plus I have also implemented the selection of themes.

This is an intermediate step, because we need to persist some information (mode language and theme) in the server-side to later use it when you save, update or call a kilink... this will take me some days and probably help because I will have to deal with the backend and the flask thing, which I do not not enough yet... 
Although it is an intermediate step, I believe we can merge it after review because add a lot of info to the users, and the same problem about persisting some info is present in the current master.

Well, test it and let me know.

Abrazo.
Damián