Summary:
Simplify UI:
 * Select option in the available browser
 * use mimeapps.list preferred service as source of truth regarding default browser
 * Can select any program as browser
 * falkon is used as default browser when installed

BUG: 100016
FIXED-IN: 5.19.0

Test Plan:
Before:

{F7888479}

After:

{F7888480}

Reviewers: ngraham, #plasma, ervin, dfaure

Reviewed By: ngraham, ervin

Subscribers: plasma-devel

Tags: #plasma

Maniphest Tasks: T12551

Differential Revision: https://phabricator.kde.org/D26731