Project: Simplfy file adding/removing interface

The filetype is only relevant for Qt4 projects. But even for Qt4 projects
the file type is insufficient to decide where the file should be added.
So remove the file type from the interface and let the projectmanagers
themselves figure out what they want to do.

Also fix
Task-number: QTCREATORBUG-9688

Change-Id: I02f7b1cd2e05efaf76e36fb9af34b109d4482f88
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>