Change StartupWMClass to com.borgbase.Vorta

[gpenz]

Description

Change the StartupWMClass desktop entry from python3 to com.borgbase.Vorta.

Related Issue

#2535

Motivation and Context

Explained in #2535

How Has This Been Tested?

I changed the desktop file locally, logged out and back in, launched Vorta and checked its icon was fine.
Opened a python app that has wmclass=python3 (Universal Radio Hacker) and checked its window wasn't eaten by Vorta.
Repeated a couple times with the change (no bug) and without the change (bug).
Environment: GNOME Wayland, version 50.4, Arch Linux.
It shouldn't affect other areas of the code, however if the wmclass of the application is ever changed (it's affected by QGuiApplication.setDesktopFileName()) it should be updated here too.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have read the CONTRIBUTING guide.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

I provide my contribution under the terms of the license of this repository and I affirm the Developer Certificate of Origin.