Let the user choose main class, when none is open

[jlahoda]

In the VS Code, when attempting to start a program using the Run pane, without any file opened, the Run fails, as no main class is found.

This patch proposes to show a dialog so that the user can choose a main class. In case only one main class is found, it is selected automatically.

The intent behind the getServerLookup() -> getServerServicesLookup() change is to ensure the content of the default lookup is present in context.getLspSession().getLookup() only once. It is added there once in the session lookup, and is also part of the server lookup. So, the patch is instead using a subset of the server lookup that does not include the default lookup.

---

^Add meaningful description above

By opening a pull request you confirm that, unless explicitly stated otherwise, the changes -

• are all your own work, and you have the right to contribute them.
• are contributed solely under the terms and conditions of the Apache License 2.0 (see section 5 of the license for more information).

Please make sure (eg. git log) that all commits have a valid name and email address for you in the Author field.

If you're a first time contributor, see the Contributing guidelines for more information.

If you're a committer, please label the PR before pressing "Create pull request" so that the right test jobs can run.

[dbalek]

Looks fine.

[mbien]

@dbalek @sdedic I added LSP label and going to restart tests so that LSP tests are run. Labels are also important for getting this PR into the release notes. Please check those things before approving to make it less likely that the master is red (which just happened a week ago, so this isn't theoretical - although I have no doubt that jan tested this PR properly).

see wiki for more details

[matthiasblaesing]

@jlahoda this is reviewed - any reason not to merge?

[jlahoda]

I realized yesterday I'd like one more tweak here (more appropriate place to set the lookup), I'm working on that.

[mbien]

would recommend to squash before merge.

btw @jlahoda would you be ok with #5617, since we will probably turn off the github squash feature, which would require to squash locally as last step before pressing merge.