Summary:
When the auto start entry is first loaded, the module only take into account
the "Name" entry in the desktop file. However, after editing or when adding
a new one, `KService::name` is used which uses the file name as a fallback.
This causes the name to "mysteriously" appear and disappear when
checking the setting or reopenning the setting page.

This change fixes the issue by using the same function `KService::name`
in all cases. This is less efficient but easier to maintain than
duplicating the logic from `KService`.

Also use the newly available `KService` instance to replace some of
the manual reading of the desktop file.
The handling of `NotShowIn` and `OnlyShowIn` are left in place since
`KService::showInCurrentDesktop()` takes into account the current running
environment rather than only `KDE`.
Changing that behavior should be a separate issue.

Test Plan: Tested on 5.18.3. Unable to test on master (5.18.80)

Reviewers: apol, davidedmundson

Reviewed By: apol, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28113