Add UI owner handle

[Silex]

Description

This adds the concept of UI owner handle so GUIs using MilestonePSTools can do proper modals.
It also unify this using helpers so the .TopMost fallbacks are handled centrally instead of at various places.

Motivation and Context

I'm using MilestonePSTools from a WPF app and my other UI helpers use this UI owner handle pattern so my dialogs are modal to the main window.

How Has This Been Tested?

Untested of course 🤣

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.