Make Konqueror a KUniqueApplication, as QtWebEngine expects.

Using QtWebEngine (with the same profile, i.e. local directory
for cookies and visited links history etc.) from several processes
hits many issues (file corruption, asserts, etc.).
I actually fixed one such issue (https://bugreports.qt.io/browse/QTBUG-59244)
but I doubt it will ever get upstreamed into chromium...
Easier to just do like other webengine-based browsers and use a single GUI process.