allow pinning PIP to one workspace

### Problem

Sometimes I setup my workspace with bunch of windows and some PIP window (with graph, PPT slide, etc). Then I want to do something on another workspace (change music, respond in chat, duckduck some help online, etc) and the PIP is getting in the way.

**Note:** I'm still on Hera, too busy to update to Odin (also some apps are still missing there), so I have the old behaviour of PIP. Still didn't get my hands on the [new design](https://github.com/elementary/gala/pull/1176).

### Proposal

I think PIP window could have one more button with a pin, to allow pinning it to current workspace only.

This problem could be alternatively addressed by making it easier for PIP to get out of the way by other means. There are some proposals for that (e.g. https://github.com/elementary/gala/issues/1175). I am opening this ticket also to show a possible new use case for those designs to consider.

### Prior Art (Optional)

_No response_

The issue names no files, tests, or entry points. Start by reviewing the current PIP and workspace behavior, then define how pinning a PIP window to the current workspace should work and verify it against the proposal and linked alternative designs.

Written by the indexing model from the issue text.