Move HDY_STYLE_PROVIDER_PRIORITY_OVERRIDE to hdy-main.c
This allows removing hdy-style-private.h, as there is no point in keeping it if it's only included by hdy-main.c and only contains a macro definition.
This allows removing hdy-style-private.h, as there is no point in keeping it if it's only included by hdy-main.c and only contains a macro definition.