fix(flatpak): fix broken desktop file, icons, and service

[ReenigneArcher]

Description

Fixes desktop file for flatpak (and probably Linux in general). Flatpak icons were also excluded because they did not start with the flatpak app id. Finally, the service file requires a flatpak run ... command.

Screenshot

Issues Fixed or Closed

• Fixes Flatpak application does not run and has an invalid icon #3560

Type of Change

• 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 not work as expected)
• Dependency update (updates to dependencies)
• Documentation update (changes to documentation)
• Repository update (changes to repository files, e.g. .github/...)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the in code docstring/documentation-blocks for new or existing methods/components

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 11.66%. Comparing base (3048e6f) to head (77a88d7).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #3561     +/-   ##
=========================================
  Coverage   11.66%   11.66%             
=========================================
  Files          93       93             
  Lines       17304    17304             
  Branches     8089     8089             
=========================================
  Hits         2018     2018             
- Misses      12806    14692   +1886     
+ Partials     2480      594   -1886     

Flag	Coverage Δ
Linux	11.35% <ø> (ø)
Windows	13.09% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/system_tray.cpp	0.00% <ø> (ø)

... and 30 files with indirect coverage changes

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud