Fixes#2381
Opening a Maven library class without a sources jar showed decompiled code with no way to get the real source. The decompiled view's banner now offers Download Sources, which resolves the artifact's -sources.jar into the local repository through the project's Maven launcher and repositories (maven-dependency-plugin get, non-transitive), with progress, cancellation and errors in the banner, then reopens the class at the same declaration; Choose Sources… attaches a jar through the existing library source archives setting. Declaration matching after reopening now skips calls and comments. Gradle downloads are not covered yet.