Guides API: add functionality to get and set y coordinate in the post-1.0 coordinate system

What does the merge request do?

• Adds a new attribute position to the guide, which is the position in the post-1.0 coordinate system (y pointing downwards, zero at the top left corner of the viewbox)
• The old attribute point is deprecated, if the raw position is wanted, explicitly use Guide.raw_position
• NamedView.new_guide, NamedView.new_unique_guide and Guide.move_to have been deprecated. This is a preparation for a new guide element in Inkscape which is defined in the user coordinate system. NamedView.new_guide, NamedView.add_unique_guide and Guide.set_position are the replacements and use the new coordinate system.
• Angles / normal vectors are also evaluated in the post 1.0 coordinate system (i.e. clockwise angles). The new property Guide.angle returns the angle in the post-1.0 coordinate system in degrees.
• All extensions reading and creating guides have been checked if the API usage can be simplified (and deprecated calls removed)
• Added very extensive docstrings. This might be a little bit exaggerated, but I'd rather document this change properly.

Brought up by @Moini in !480 (comment 965090094)

Summary for release notes

The Guides API has been rewritten to get and set guide properties in the post-1.0 coordinate system, as the current guide API is rather confusing in a multipage setting. For this, Guide.point has been deprecated in favor of Guide.position (new coordinate system) and Guide.raw_position (old coordinate system). Also, NamedView.new_guide, NamedView.new_unique_guide and Guide.move_to have been deprecated in favor of NamedView.new_guide, NamedView.add_unique_guide and Guide.set_position.

Checklist

• Add unit tests (if applicable)
• Changes to inkex/ are well documented
• Clean merge request history