KDE4: Don't use native file pickers per default.

Add a configure option to build with native KDE4 file pickers and
disable them. Even with all the workarounds applied it's impossible
to work around the bug in the Qt4 glib event loop, which doesn't
ignore socket events when polling the clipboard through QClipboard.

See https://bugreports.qt-project.org/browse/QTBUG-37380.

Change-Id: I5cad30ead74571e49a075c084cca7a19acff7523