This change introduces a new theme configuration struct to be
passed to the aforementioned routines to better control reuse
of styles and definitions in the generator.
Everything not passed and not explicitly shared is scoped so it
is not automatically accessible throughout the routine. This
should ensure that the decision to share styles is a conscious one.
In the future we will try to reduce the number of unique definitions
and share most of it. This PR is a stepping stone on this path.
This also puts the effort into separating redefinitions of
default theme items vs custom types introduced only by the editor.
In a few cases where editor-specific definitions need to reference
default definitions we simply fetch them from the theme. It's not
ideal and hides the dependency a bit, but hopefully these cases
will be abstracted properly in due time.
635b8a1474