Trying to load gigantic directories *before* accessing them

[EmmanuelMess]

Take for instance a directory of 1500 files (could happen) or more, MainFragment will simply find every file in it, and then show the directory, in my Moto E2 4G that takes time. I propose loading dynamically and telling the user that the directory isn't fully loaded; take also into account that the user won't look at each (1500) file/directory inside, probably they will just search.