Missing dependencies for DEB Linux package and app launcher

[henriquead7]

There is no app launcher (shortcut icon) in the package.

In version 0.8.3 for Ubuntu 20.04 (I haven't checked the others), an OpenGL-related lib (libopengl0) is missing. Impossible to run the app.

I modified the mgba-qt.deb package by adding the missing dependency and the app launcher. It worked perfectly, but other users may have problems with it.

[endrift]

I should mention, the reason I haven't fixed this yet is because I haven't figured out if mGBA is supposed to depend on libOpenGL or if I'm doing the linking wrong with newer CMake. I'm not intentionally linking it; it's an artifact of GLVND support in modern CMake, but I haven't quite figured out what changes I need to make as a result yet.