Don't trigger code action requests for background views

[rchl]

Servers like typescript trigger textDocument/publishDiagnostics for all open files on making any changes. Our DocumentSyncListener triggered code action request in that case.

Changed so that code action request is only made if the view is visible.

Since the code action response can be affected by changed diagnostics, make sure that the code action request is then also made when view is activated later.

We should have been triggering code action request on view activation anyway as otherwise we have relied on textDocument/publishDiagnostics triggering it which is not guaranteed.

Also made sure that on_load_async triggers on_activated_async which triggers code actions/code lenses/semantic tokens because when file is initially focused the view is not registered yet so we have missed all the logic currently in on_activated_async.

Fixes #2101

[rchl]

Will take no feedback as "no issues" :)