ProWriter: Prepend $$PWD to files if adding to a .pri file

QMake's behavior for .pri files otherwise treats those paths as relative
to the .pro file. That's rarely what you want.

Also change the remove code to handle those files.

Change-Id: I5fe95035b470c32c365adbdee161f4a6cae05c2b
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>