Fix option for diff executable: allow plain names, use "" as default

Summary:
Code in libkomparediff & kompare assumes that m_diffProgram.isEmpty()
means default, so setting an explicit "diff" on "Defaults" button click
screwed that.
Even more did QUrl::fromLocalFile/QUrl::toLocalFile not play well for
people just entering the straight name of the executable without a path.

Test Plan:
Tested with copies of diff exectuable in custom paths, either given with
fill path or just by name as found in PATH, also after program restarts.
Resetting to Defaults also works as expected.

Reviewers: cgiboudeaux

Reviewed By: cgiboudeaux

Differential Revision: https://phabricator.kde.org/D20850