Use folder prefix is filename is not unique

Summary:
If several files have the same filename in the open document menu,
prefix such with their containing folder name. This changes e.g.

• CMakeLists.txt
• CMakeLists.txt

to

• foo/CMakeLists.txt
• baa/CMakeLists.txt

Test Plan: Manual testing

Reviewers: kfunk, KDevelop, apol

Reviewed By: KDevelop, apol

Subscribers: apol, kfunk, brauch, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D1947