Improve metadata to follow Flathub guidelines

[AlexanderVanhee]

Hello,

I saw that this project was fairly close to meeting the Flathub metadata quality guidelines. Only three easily solvable issues were preventing it from passing:

• The screenshots didn’t have shadows (this is an issue with Qt apps on GNOME, so I took them on KDE, where shadows are included).
• The summary was too long.
• The app was missing brand colors.

Please make sure to request a re-review on Flathub after this has been included in a release.

[grulja]

Thank you!!