When opened in SUI, Command Palette is full of actions that can't be used in the SUI

> The only complaint I have is that the command palette is filled with some actions that don't do anything (i.e. "split pane"). It'd be nice to filter out the ones that don't do anything, but that's definitely complicated enough to be a follow-up. Mind filing that?

This is me filing that.

We've previously discussed at length having a "context" for where actions are applicable. E.g.

* copy/paste/find are "control" actions
* togglePaneZoom is a "pane" or Page level action
* newTab is a Page? Tabs? level action
* closeWindow would be a page? level action

So we might need a way of knowing when an action is applicable, so we only show the applicable ones. may also help for code synthesis - for an action that's control level, we may be able to have some common code template for "in the context of the active control, _do the thing_".

Start by tracing the SUI command palette's action list and the existing action definitions; the issue names no files or tests. Done means actions are filtered according to their applicable context, with the context model covering examples such as control-, pane-, page-, and tab-level actions.

Written by the indexing model from the issue text.