Improve file download path management for desktop

[Te-Z]

On desktop the downloaded files by the users are stored in the temporary directory (for example if I want to download a .pdf shared in a room).
At temporary solution has been proposed: https://github.com/linagora/twake-on-matrix/blob/91419cffde7bbf0a6661589edbce39f02b82bf1d/lib/utils/manager/storage_directory_manager.dart .

This solution allow the app to save those files in download_directory/Twake Chat/, same as Telegram for example.

Other files are still saved in the /tmp folder which is the cache temporary directory.

Also on linux, files in temporary directory are deleted

The temporary solution should be refactored using abstraction to handle the case of different platforms.

Demo:

330388647-cda8c13c-6056-4d4e-9235-b697e01c5ed2.webm

[pm-McFly]

Desktop target is not the priority yet.

Point is relevant but not urgent