Per monitor dock position

### Feature Description

Dock have 4 positions. On a multi-monitor scenario, the dock can sometimes be difficult to arrange. (think side-by-side monitor, with the dock on the side. The will be one monitor's dock that's difficult to trigger.)

### Use Case

_No response_

### Compositor(s)

All compositors

### If Other, please specify

_No response_

### Proposed Solution

Per monitor dock position selector resolves this.

### Alternatives/Existing Solutions

_No response_

No files, tests, or implementation entry points are named. Start by locating the dock positioning and monitor-handling entry points in the shell, then determine how a per-monitor selector should behave across all listed compositors; done means each monitor can use its own dock position.

Written by the indexing model from the issue text.