Session file parser: Support sections and respect the Hidden property

Summary:
Some desktop files have multiple sections, but for now we're only
interested in [Desktop Entry]. Without this patch, every entry was seen
as part of the [Desktop Entry] session, resulting in values getting
overwritten.

Additionally, the Hidden=true property specifies that the desktop file
needs to be treated like it was non-existant.

Same as https://github.com/sddm/sddm/pull/821 for sddm.

BUG: 381982

Test Plan:
Installed the KCM, now there are no duplicate sessions and the right
Name is shown for icewm-session.desktop.

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D6626