Add titlebar height setting sample

[InfyniteHeap]

Description

Provide a sample to demonstrate how to set the height of titlebar after extending contents into titlebar.

Motivation and Context

WinUI supports to set titlebar height, but the TitleBar page doesn't contain a sample to that.

How Has This Been Tested?

Just build and run this app, and trun to TitleBar page, and then test whether the combobox works as expected.

Screenshots (if appropriate):

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)

[Jay-o-Way]

Is this based on on the new experimental titlebar control (coming hopefully in WASDK 1.7) maybe? I think there's an issue with the phrasing like "... now allows ..." It is unclear what "now" means: is it a SDK version, a specific date or something else? And if it relies on a SDK version, this must be clearly mentioned.
Also, after a while - "now" will become outdated at some point in time.

Besides that: try and keep redundancy to a minimum and use normal sentence casing (see "Titlebar customizations")

[karkarl]

/azp run