Reduce amount of workspace serialization happening (#22730)

Part of https://github.com/zed-industries/zed/issues/16472

Reduces amount of workspace serialization happening by:
* fixing the deserialization logic: now it does not set panels that are
hidden to active
* cleaning up `active_panel_index` for docks that are closed, to avoid
emitting extra events for such closed docks
* adjusting outline panel to drop active editor subscriptions on
deactivation — this way, `cx.observe` on the dock with outline panel is
not triggered (used to be triggered on every selection change before)
* adjusting workspace dock drag listener to remember previous
coordinates and only resize the dock if those had changed
* adjusting workspace pane event listener to ignore
`pane::Event::UserSavedItem` and `pane::Event::ChangeItemTitle` that
seem to happen relatively frequently but not influence values that are
serialized for the workspace
* not using `cx.observe` on docks, instead explicitly serializing on
panel zoom and size changes

Release Notes:

- Reduced amount of workspace serialization happening