project pluign: make the "list" mode work again

Now kate checks whether the entry in the list of files is a relative or absolute path and handles it accordingly. Before it was simply appending every entry from the list to the project directory, which resulted in wrong paths if the entries were already absolute (as is the case for projects generated by cmake)

It checks using QFileInfo. There was already a QFileInfo created before, this is now used for that, i.e. no additional QFileInfo objects are created.