fixes #3811
With this PR consumers can use the newly added Inherit member for the PrimaryColor and SecondaryColor enum.
Just like with BaseTheme="Inherit" inherit means "take the OS preferences".

This is largely based on how it is done in ControlzEx

Notes:
In the case the value could not be read from the registry, or the conversion (registry value --> C# Color) is somehow failing, the code falls back to SwatchHelper.Lookup.First().Value. One alternative would be to throw an error instead, but I don't think such an "irrelevant" feature should throw an exception.

Example:

<materialDesign:BundledTheme BaseTheme="Inherit"
                             ColorAdjustment="{materialDesign:ColorAdjustment}"
-                            PrimaryColor="DeepPurple"
+                            PrimaryColor="Inherit"
-                            SecondaryColor="Lime" />
+                            SecondaryColor="Inherit" />

I also refactored the BundledTheme class slightly.