WIP: fixes for flatpak build

[mwilck]

Hi Hervé

I've been trying to make the :flatpak:buildFlatpak work, so far without success.
This PR (DRAFT DRAFT, don't apply as-is) shows some of the fixes I made.

It's well possible that some of the commits are wrong. But I couldn't figure out better ways to overcome the various errors I encountered.

Feel free to ask back what I was trying to fix with the individual commits. I'll try to explain.

The last issue I encountered is that gradle (in the flatpak environment) doesn't find all dependencies for the schemas. It seems that schemas/build.gradle would also need a flatpakGradleGenerator task, and that this needs to be properly pulled in by flatpak/build.gradle. But that exceeds my basically non-existing gradle knowledge by far.

> Task :schemas:compileJava FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':schemas:compileJava'.
> Could not resolve all files for configuration ':schemas:compileClasspath'.
   > Could not find org.codehaus.mojo:jaxb2-maven-plugin:2.5.0.
     Searched in the following locations:
       - file:/run/build/audiveris/dependencies/org/codehaus/mojo/jaxb2-maven-plugin/2.5.0/jaxb2-maven-plugin-2.5.0.pom
     Required by:
         project :schemas
   > Could not find org.apache.maven.plugin-tools:maven-plugin-annotations:3.5.1.
     Searched in the following locations:
       - file:/run/build/audiveris/dependencies/org/apache/maven/plugin-tools/maven-plugin-annotations/3.5.1/maven-plugin-annotations-3.5.1.pom
     Required by:
         project :schemas