[Feature] Fill gaps in core windows services (WinUIEx)

### Problem

Many gaps (window size DPI aware, systray, etc.)...

### Proposed solution

With @dotMorten's guidance, we should leverage https://github.com/dotMorten/WinUIEx and fill gaps. Eventually these should merge down into WinUI (as will many other features in this repo). Don't forget property attribution in thirdpartynotices.txt.

### Alternatives considered

_No response_

### Additional context

_No response_

### Confirmation

- [x] I have searched [existing issues and specs](https://github.com/microsoft/microsoft-ui-reactor/issues?q=is%3Aissue) for prior discussion of this idea.

Start by reviewing the gaps named in the issue, such as DPI-aware window sizing and the system tray, alongside the referenced WinUIEx project. Check thirdpartynotices.txt for the required property attribution. The work is complete only when specific missing services have been implemented and the relevant attribution is present, but the issue does not define that scope.

Written by the indexing model from the issue text.