[netcore3] Ensure theme building works properly on netcore3

Copied from VSO ID 653435

The current theme setup in NetCore3 contains separate directories per theme "PresentationFramework-{ThemeName}".

In these directories under the theme directory is the generated output. At current, this is copied from the latest output in netfxdev1 and there is no generation step.

Rebuild the generation step using the shared XAML files in Shared/Themes/XAML in order to ensure the themes are building properly.

This is potentially to be done via T4, but may require re-use of the old Perl scripts if T4 proves prohibitive.