[UI] Default X11 Icon On Linux

[Jazztache]

Issue: Slippi uses the default X11 Icon on the AppImage for Linux. This is the case for both the taskbar and the KDE Plasma window overview
Solution: Set the icon to use the same monochrome icon as in the top right corner. Alternatively, you could use a higher resolution version of the slippi.gg favicon.

Thankyou!