Improve Java support in QtCreator

Currently most waxing issues with Java are:

1) only basic Java syntax highlighting available - no code completion, error-checking and other handful features.
There is an theoretical possibility to setup Java LS (https://doc.qt.io/qtcreator/creator-language-servers.html) but on practice i don't managed to setup it or find an guide with necessary details. So, it would be good to see more precise documentation on setting up Java LS or to have Java LS working 'out of box'.

2) weird messages at issues tab for java compilation errors (single actual error message interpreted as multiple junky issues (https://bugreports.qt.io/secure/attachment/137568/weird_messages_at_issues_pane_view_java.png).

3)TODO (experimental) plugin don't support Java files (QTCREATORBUG-28695)