KCM/Component Revamp email config

Summary:
Simplify UI:

• Select option in the available email clients
• use mimeapps.list preferred service as source of truth regarding default email client
• keep kmail as default email client when it is installed
• Can select any service as email client alternatively
• Ensure kbuildsycoca cache is updated after saving

BUG: 292606
FIXED-IN: 5.19

Test Plan:
Before:

After:

Reviewers: ngraham, ervin, Plasma, bport, crossi, dvratil

Reviewed By: ngraham, ervin

Subscribers: plasma-devel

Tags: Plasma

Maniphest Tasks: T12551

Differential Revision: https://phabricator.kde.org/D26565