Window-less Managed File Dialog

[maxkatz6]

What does the pull request do?

This PR is supposed to replace #11595, and improve some flexibility about our managed dialogs.
Technically, it's still not supported on mobile (and shouldn't), but developers now can setup

1. Previously, Managed File Dialog was creating a window to host dialog. Now it will use Popup, when window is not possible.
2. Enables nullable attributes on Dialogs project
3. Extracts ManagedFileChooserOverwritePrompt into a separated control with styling. Previously its creation was hardcoded in C#.
4. Adds BclMountedVolumeInfoProvider as a fallback, when better IMountedVolumeInfoProvider is not specified.
5. Adds SupportedOSPlatform to support only windows, linux, macos. Other platforms are technically possible, but with warning.
6. Extends ManagedFileDialogOptions to allow custom IMountedVolumeInfoProvider and root view factory.

Tested on Windows (with some modifications to force Popup implementation) and Android.
As expected, on android BclMountedVolumeInfoProvider returns junk volumes, and requires extra permissions. It is possible to implement Android-specific IMountedVolumeInfoProvider by the developer.

Breaking changes

No

Fixed issues

Closes #11595 (alternative solution)
Fixes #8437

[jmacato]

Tested LGTM!