Make sure caption controls "dim out" when window loses NC focus

[AnuthaDev]

Hello frands, I gib smol code

Summary

Make the caption controls "dim out" when window loses focus

References

https://docs.microsoft.com/en-us/windows/win32/winmsg/wm-ncactivate
microsoft#5881
microsoft#3025

PR Checklist

• Applies to Epic: Fix remaining issues with non-client drawing microsoft/terminal#1625
• CLA signed. If not, go over here and sign the CLA
• Tests added/passed
• Requires documentation to be updated
• I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

Detailed Description of the Pull Request / Additional comments

Added a handler for WM_NCACTIVATE to propagate the focus change to the Titlebar Control.

The unfocused color is set to ForegroundBaseLow

Validation Steps Performed

None
Typed code on phone, PC is too weak to compile the whole project.

Also, My combined knowledge of c++ and xaml is still miniscule, so please have mercy😅😅