Handle folders

In some courses, teacher make use of «folders» containing few files. Sadly, we need to make another request to know what's in a folder.

GuiResource is to be the one managing this stuff, but it still raise an issue about how we will integrate the retrieved resources in the list. It is possible to use a GtkExpander with a GuiResource as label, but for that, we need to change the base class of GuiResource from HdyActionRow to something else.