More dictionaries and new language selector

After a lot of internal work, providing new dictionaries and updating existing ones will be much easier from now on. As a result of this, dictionaries between all 11 supported languages are available now. Since the large number of 55 dictionaries (lang1->lang2 and lang2->lang1 counted as one) made it rather hard to select the desired dictionary, I've redone the dictionary selection on the home page and in the top menu:

Before (with just 15 dictionaries):

 Now (with 55 dictionaries):

As you can see, the dictionaries on the home page are grouped in drop-down menus by language now, which is much more compact than the previous presentation.

The drop-down in the menu bar still contains the recently selected dictionaries. But the full list of dictionaries below that has been replaced by lists with dictionaries for the two currently selected languages. This allows quickly switching one of the selected languages for another.