KCM/Component Revamp Browser config

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:

After:

Reviewers: ngraham, Plasma, ervin, dfaure

Reviewed By: ngraham, ervin

Subscribers: plasma-devel

Tags: Plasma

Maniphest Tasks: T12551

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