Include updates for DMS-Plugins inside the Systemupdate widget

### Feature Description

The Systemupdate widget should include updates to installed DMS-Plugins.

### Use Case

I myself did not realize i had pending updates to Plugins for weeks. I think other users will expect the DMS-Systemupdate to include plugin updates aswell.

### Compositor(s)

All compositors

### If Other, please specify

_No response_

### Proposed Solution

_No response_

### Alternatives/Existing Solutions

The "Dank Software Depot" plugin already includes plugin updates. Still i think it should be a feature of the native tool

Start at the Systemupdate widget entry point and compare its current update sources with the “Dank Software Depot” plugin, which already includes plugin updates. Confirm how installed DMS-Plugins expose pending updates and define completion as showing those updates in the native widget for all compositors; the issue names no files or tests, so locating those entry points is the first task.

Written by the indexing model from the issue text.