Use QDesktopServices to open urls not supported by dolphin

Summary:

This improves portability.
QDesktopServices uses standard mimetype base resolution to find the app to use.

BUG: 372642
FIXED-IN: 20.04

Test Plan:
 * Open http://google.com, opened default web browser
 * Open mms:// (on my test system opened vlc)

Reviewers: #dolphin, elvisangelaccio, ngraham

Reviewed By: #dolphin, elvisangelaccio, ngraham

Subscribers: nicolasfella, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D26732