Added WindowManagerAddShadowHint to PopupRoot.

[grokys]

What does the pull request do?

Previously, the WindowManagerAddShadowHint property was defined on Popup and ContextMenu but not PopupRoot: instead various controls such as the aforementioned two and ToolTip used the PopupRoot.PlatformImpl.SetWindowManagerAddShadowHint internal method to set the "add shadow" hint. This had the downside that 3rd parties could not set this property when using PopupRoot directly.

This PR adds a WindowManagerAddShadowHint property to PopupRoot and makes the controls that previously used the internal interface use that property instead.

[grokys]

This isn't ideal but the default value of false for this property appears to be different to the default value implemented in the win32 backend (true), so this is needed to communicate the actual default. I could change the backend but didn't really want to mess with that here quite honestly.

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0046957-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[maxkatz6]

I would argue that third parties should not use PopupRoot ever. And we should make Popup api better (as well as rebase Tooltips on top of Popups).

But this PR is reasonable by itself.

[grokys]

I would argue that third parties should not use PopupRoot ever. And we should make Popup api better (as well as rebase Tooltips on top of Popups).

Yeah, probably. But at least with 11.x PopupRoot is a public API so this PR makes sense I think. We should start by migrating our internal usage of PopupRoot to Popup I guess.