Add desktopFileName static methods to QGuiApplication

[dphaldes]

Added methods :

fn set_desktop_file_name(name: &QString);
fn desktop_file_name() -> QString;

This is required to set the application desktop name association on Linux. This allows desktop entries and taskbar icons.
Example:

Example usage: https://github.com/mystchonky/kontrast-rs/blob/54ace175a97f044c714599bcbf645e95a766c8a0/src/main.rs#L14

[dphaldes]

I have tried to follow the code structure/organization. I can move the functions around if something else is more suitable

[LeonMatthesKDAB]

Hi @mystchonky ,

Thank you for the contribution. The change looks good 😊

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (dff128f) to head (8e26aaa).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1087   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           71        71           
  Lines        11928     11928           
=========================================
  Hits         11928     11928           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.