AppImage: Include wayland platform plugins

[galister]

When running under wayland, the xcb pluging is trying to connect to XWayland, but for some reason it's unable. I didn't really look into the why. Other Qt5 apps seem to have no such issue.

What I did end up doing is build and AppImage with the wayland platform plugins included. This is really simple and can be done in only 2 small steps:

Installing the qt wayland plugin (qtwayland5 on ubuntu)

And then adding the following to linuxdeployqt

-extra-plugins=platforms/libqwayland-egl.so,platforms/libqwayland-generic.so

(or otherwise copying these 2 binaries from the plugins/platforms folder)