Add optional utility widgets for the Dock

### Feature Description

Add optional utility widgets that can be shown in the Dock next to app icons.

Examples:

- Color picker
- Workspace preview / switcher
- Screenshot tool

These should be configurable so the Dock stays simple by default.

### Use Case

Some actions are useful enough to access directly from the screen edge, but do not always need a full bar widget or Control Center entry. Optional Dock widgets would make those actions easier to reach without changing the default Dock behavior.

### Compositor(s)

All compositors

### If Other, please specify

_No response_

### Proposed Solution

Add a Dock widgets setting section where users can enable and reorder small utility buttons independently from pinned apps.

Initial candidates could be color picker, screenshot, and workspace preview.

Example in Plank Reloaded:

### Alternatives/Existing Solutions

Bar widgets, Control Center actions, launcher commands, external tools, or compositor keybindings can already cover this, but they are less direct for mouse-first Dock workflows.

The issue names no files, tests, or entry points. Start by locating the Dock settings and pinned-app configuration, then determine how optional widgets can be enabled and reordered across all compositors. Done means configurable color picker, screenshot, and workspace preview candidates can appear independently while the default Dock remains unchanged.

Written by the indexing model from the issue text.