Test for mismatch between .desktop file and the WM_CLASS that the window reports

Test for mismatch between .desktop file and the WM_CLASS that the window reports and/or missing `QApplication::setWindowIcon()` (i.e., no icon on the dock)

We can find out which WM_CLASS your window has got when you type in a terminal: `xprop WM_CLASS` and then click at the window.

Some applications need this in the desktop file, like so:

```
StartupWMClass=discord
```

References:
http://discourse.appimage.org/t/appimage-does-not-show-icon-in-task-bar/40/5
https://askubuntu.com/questions/367396/what-represent-the-startupwmclass-field-of-a-desktop-file
https://solus-project.com/forums/viewtopic.php?t=9729