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:

{F7883144}

After:

{F7883141}

{F7883117}

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