Allow foldernodes to specify which files they care for when doing
"Add Existing Directory". Before we used a fixed list with C++ header
and source files, which does make no sense for e.g. adding to resources.

Task-number: QTCREATORBUG-15278
Change-Id: I15dad133391485c2bcebd2d864623304b31b5f8f
Reviewed-by: Niels Weber <niels.weber@theqtcompany.com>