add default icon for games without icon when creating desktop shortcut

[soredake]

Describe the bug

Creating desktop shortcut for game without icon results in shortcut without icon.
Sample game https://lutris.net/games/sonic-classic-heroes/

Steps to reproduce

• Install lutris from ppa on KDE.
• Install game https://lutris.net/games/sonic-classic-heroes/
• Create desktop shortcut.

Lutris debugging output (Optional)

lutris.zip

System information (Optional)

lutris-issue-report.zip

Screenshots (Optional)

[Capital2]

The title of the issue and its content are 2 different issues.
Your problem seems to be with the DE's icon theme and not related to lutris:
Replicated the problem on KDE, and the default "Icon not found" icon showed up.

The title suggests that the aforementioned default DE icon is not wanted and as an example putting the lutris icon as a replacement on icon-less launchers, if that is the case please consider updating the issue.

[soredake]

I can't reproduce this with recent KDE, as for setting lutris icon to entries without own icon i will open another FR.