Previously, the Shaders, Texture packs and Resource packs tabs had as
parent the ModFolderPage, making it so that making changes only to the
Mods page would require checking the id of the page for the correct one.
This was hackish and error-prone.
Now, those pages all inherit from a single class, ExternalResourcesPage,
that handles the basic behaviour of all of them, while allowing for
individual modification in code.
This is still not a clear separation, since internally, all those
resources are derived from Mods, so for now there's still some awkward
common code :/
