Add MIME type for Step files

Summary:
Currently, Step files are just generic XML files for Plasma. This patch adds a MIME type for them and let Step to be a handler for .step files.

BUG: 296317

Test Plan:

1. Install Step.
2. Open Dolphin in the Step examples folder (/usr/share/step/examples/). All .step files should be shown with their icons.
3. Double click on any file. Step should be opened with the clicked example.

Dolphin with .step files:

Reviewers: KDE Edu, aacid

Reviewed By: aacid

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D22873