Followups to Parent Pane Focusing

# Description of the new feature/enhancement
https://github.com/microsoft/terminal/pull/11153 adds the ability to focus multiple panes at the same time. While it implements many actions for parent panes, there are still some things that can be improved
- [ ] When multiple panes are selected, the tab icon could be the "split pane" icon.
- [ ] Searching currently only works on one pane at a time.
- [ ] Broadcasting input to multiple panes? The `scroll*`/`send-input`/`copy`/`paste` actions and general key presses could possibly have a well-defined interaction with multiple panes, but that will probably be an implementation detail for https://github.com/microsoft/terminal/issues/2634
- [ ] [Explore Idea] When zooming a parent pane, it might be nice to move around within the zoomed parent pane without it changing the zoomed pane.
- [ ] [Explore Idea] `moveFocus`: if we have a subtree of panes selected, maybe we should maintain the size of the subtree and focus another group of panes?
- [ ] Check everything else that still uses `GetActiveControl` to see if it could/should be updated to updating all controls.
- [ ] The duplicate pane option could duplicate all selected panes instead of just the one that has focus. Similarly once this is implemented it should be trivial to make duplicate tab create an entire copy of the tab instead of just the terminal with focus.
- [ ] #11359

Start with PR #11153 and the checklist in this issue; no files, tests, or narrower entry points are named. The linked issue #2634 and reference #11359 provide related context. Choose one follow-up and establish focused acceptance criteria and regression coverage, since the current list mixes implementation tasks with exploratory ideas.

Written by the indexing model from the issue text.