Clear merge debris (Re: #6416)

[teunbrand]

This PR re-implements #6416 on rectified main branch.

It largely shares the same goals:

• Adds element_geom(), element_polygon() and element_point() to is_theme_element(type) options.
• Consistently use the new is_*() functions.
• Convert remaining internal is.*() functions to is_*() functions.