Add keywords to .desktop file

Summary:
FEATURE: 400989

The .desktop file for Spectacle doesn't have a "Keywords" entry. It would probably do well to provide Spectacle as a result when users search for 'Snipping Tool' as that's the Windows variant.

Test Plan: desktop-file-validate both before and after applying the patch

Reviewers: #spectacle, ngraham

Reviewed By: #spectacle, ngraham

Subscribers: ngraham, pino, #spectacle

Tags: #spectacle

Differential Revision: https://phabricator.kde.org/D17396