Closes#1949. Plugin API 1.3: views from several plugins can share one tool window (shown as tabs) and a view can declare a visibility condition (e.g. only when the project contains a file, or for the active file) so it appears only when it applies. Additive within API 1.x, with a fixture plugin, regression tests and docs.
Co-authored-by: arnaud-ritti <[email protected]>
