windows: add support for system theme

Use registry
"HKEY_CURRENT_USER/Software/Microsoft/Windows/CurrentVersion
/Themes/Personalize/AppsUseLightTheme"
to check if system theme is supported and if it is dark or
light.

Removes "EnableDarkTheme" in favor of "AppTheme".

Requires Windows SDK version 10.0.18362.0 to build with
system theme support.

Note: This does not watch for changes in system theme in
runtime as the support for it requires Windows Runtime
version 10.0.10240.0.

GitLab: #723

Change-Id: Ice8f7936a90535f47dc1870d4f18215e062684ba