[Feature Request] Sublevels for Custom Collections

[elmimmo]

Please, add support for nested Custom Game Collections, in order to recreate rich and deep collections such as the Game Series Collections by tjstogy.

Just as a possible idea of how to implement it in the file tree, MinUI, which also supports Collections via text files with rom paths, allows subdirectories in its Collections directory, as deep as one may want. Those subdirectories appear listed inside its Collections menu along with regular collection files. When subdirectories share the same name as a collection file, its contained subcollections are listed as subfolders in the same level as the roms in the collection file.

In lack of a UI to create subcollections, the UI to give a name to a custom collection could support the "/" character as a level separator, which it currently omits. Of course, it would be easier to simply allow the creation of new Collections from within the Options menu when inside any level of the Custom Game Collections category.