NumberBox header and description display issues

[Kinnara]

Describe the bug

<muxc:NumberBox
    Width="300"
    Header="Header Header Header Header Header"
    Description="Description"
    SpinButtonPlacementMode="Inline" />

There are two issues:

1. The header wraps too early, before reaching the right edge of the control.
2. The spin buttons are offset when description is used.

I think one way to fix these is to move the header and description content presenters out of the TextBox and into the NumberBox itself.

Expected behavior

Version Info

NuGet package version:
Microsoft.UI.Xaml 2.3.191211002

Windows 10 version	Saw the problem?
Insider Build (xxxxx)
May 2019 Update (18362)	Yes
October 2018 Update (17763)
April 2018 Update (17134)
Fall Creators Update (16299)
Creators Update (15063)

[StephenLPeters]

@teaP FYI

[StephenLPeters]

@SavoySchuler @chigy should the header be wrapping at all?