make kcolorschemeditor desktop entry spec compliant and refine it

Summary:
quoting the menu spec [1]:

Additional Categories should always be used in combination
with one of the Main Categories.

So, following changes:

• Settings is now the main category in the Categories entry
• NoDisplay=true as we only use it from the KCM and it would only clutter the app menu otherwise
• OnlyShowIn=KDE for good measure on top of nodisplay as the application has only value within a plasma context
• Remove bogus MimeType entry. The Exec key takes no arguments, so the mimetype association is simply wrong.

[1] https://specifications.freedesktop.org/menu-spec/1.0/apa.html

Reviewers: bshah, ochurlaud

Reviewed By: bshah, ochurlaud

Subscribers: ochurlaud, bshah, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2478