GStreamer and QT Multimedia can now be used inside an appimage
with only a few changes to the docker build environment.

Docker build environment currently:
  ~ Needs to build QT dependencies from source using changes to `3rdpart/ext_qt/CMakeLists.txt`
  ~ Needs to have GStreamer and GStreamer Plugins installed on the system.

Gstreamer specific environment variables are set within main.cc **only** when running
Krita through an AppImage context.

The appimage_build script could use some cleaning. Notable changes:
  - GStreamer is `install`ed from docker environment libraries into APPDIR
  - QtMultimedia's `mediaserver` plugin is copied into the APPDIR.
  - Most importantly -- linuxdeployqt was filtering out `mediaserver` from our appimage bundle
when building with the `-appimage` flag for an unknown reason. A simple work around for this
seems to be to simply remove the `--appimage` flag from linuxdeployqt and use `appimagetool`
on the appdir manually. This seems to fix the issues I was having with linuxdeployqt.

Future Tasks:
1 -[ ] Begin reductions on which gstreamer libraries are included on the system. This will be time consuming
but will result in a smaller appimage size. Shouldn't be needed immediately though and is a polish step.
2 -[ ] Make sure changes to docker-environment are reduced to the bare-minimum necessary to get a working build.
If we could get GStreamer binaries with wget or alternatively build gstreamer and all necessary plugins from
source, it would mean a leaner docker image to download. Building GStreamer from source might be a pain.

Changed `isInAppimage` GStreamer ENV logic to match other parts of Krita

Cherry-Pick from Krita/4.3