Revive folderpanel when outside $HOME

Summary:
Currently the folderpanel is inert when browsing outside of home because urls end up as 'file:////a/b/c'
and since Qt 5.10 this is an invalid URL: http://code.qt.io/cgit/qt/qtbase.git/commit/?id=f62768d046528636789f901ac79e2cfa1843a7b7

Premptive bug fix.

Test Plan:
Open dolphin
Leave home directory

Reviewers: #dolphin, elvisangelaccio

Subscribers: dfaure, elvisangelaccio, ngraham, anthonyfieroni

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D9610