[feature] Allow to use language folders

[martinec]

Is your feature request related to a problem? Please describe.

The IDE requires a notion of a language folder. This will avoid displaying folders that do not contain a language.

Describe the solution you'd like

The proposed approach is as follows:

Each language folder contains a file, named <language_code>.info, e.g. fr.info with the following mandatory information:

language_name = French
language_native = Français
langue_code = fr

(check out the Languages table on https://github.com/UnitexGramLab/unitex-lingua)

Then, the IDE must proposes as languages only folders containing a .info file with at least the 3 information mentioned above (language_name, language_native, language_code)

It must be possible to override this behavior by unchecking a check box "Show only language folders".

If no language file is found, the IDE may suggest to search in another location, and shall mention that other languages are available to download from https://unitexgramlab.org/releases/latest-stable/lingua/

Describe alternatives you've considered

NA

Additional context

The current dialog:

The proposed dialog: