GraphEdit connection rework (API & optimizations)

[Geometror]

Detailed changes:

• Add get_closest_connection_at_point which return the closest connection at a given point in GraphEdit's local screen space (with the given maximum search distance). See the docs for a usage example.
• Optimize connection drawing/handling
  • Introduce connections HashMap (node -> its connections)
    • The old logic iterated over all connections for every GraphNode, now only the relevant connections are checked where possible.
  • Each connection now caches the line, the start and end positions (in screen space) as well as an AABB. This allows for updating/drawing only those connections visible on the screen (this had a huge performance impact for large graphs). It's also used to accelerate fetching the connection from a screen position which is introduced in this PR.
• Adjust theming/visual appearance
  • hovering a connection now highlights it (theme item connection_hover_tint_color)
  • the highlight color when dragging a connection onto a valid port is now a theme item (theme item connection_valid_target_tint_color)
• Change signature of get_connection_list (now returns the list instead of using a return parameter) [only used internally]

For a demonstration of these new features see #83510.

[fire]

Is there a guide on how to get the point from whatever space to screenspace that the GraphEdit is in? I remember it was difficult to click on close buttons a long time ago. Maybe clicking on the graph line has the same issues.

Use case: 3d graph runner - can be arranged in 3d coordinates up oriented billboards.

Edited: I see your comment about See the docs for a usage example.

[Geometror]

Changes:

• Add method get_connections_intersecting_with_rect
• Add method reset_all_connection_activity
• get_closest_connection_at_point (and the new method) now fully transforms the screen space coordinates internally, so get_closest_connection_at_point(local_mouse_position + get_scroll_offset()) now becomes just
  get_closest_connection_at_point(local_mouse_position)

Edit: The two new methods can now be seen in action here: #83510.

[Geometror]

Superseded by #86158.