Come up with a new validation graph widget

[hgonomeg]

With the removal of goocanvas a new interactive widget to display validation graphs is needed.

Essentially this has to be a bar chart with clickable rectangles.

Approaches

1. A new GtkWidget that draws appropriate rectangles and is able to handle click events

Tasks

• [x] Setup gui for playground
• [x] Research how rendering changed in gtk4 [GtkSnapshot now abstracts away cairo]
• [x] Derive new GObject type
• [x] Draw the graph
  • [x] Label chains
  • [x] Draw chains
    • [x] Draw axes
    • [x] Draw axes' labels
    • [x] Draw residue ruler
    • [x] Draw residues
    • [x] Draw rectangles
    • [x] Normalize
    • [x] Color indication
• [x] Add handling click events
• [x] Implement finding residues based on x, y coords
• [x] Add residue-clicked signal
• [x] Add hover-handling event-controllers
• [ ] Make it possible to render two blocks per residue - "stacked mode"
  • [ ] Define how the data model represents this
• [x] Add residue tooltips
• [x] Investigate whether 'rescale' feature is needed with normalization.
• [x] Use an enum to determine the graph's data type, so that it renders its' content adequately
  • [x] Fixed min/max for probabilities
  • ~Min/max value from hint~
  • [x] Support for bars "hanging-down"
  • [x] Support for logarithmic probability
• [x] Add support for single-chain display mode
• [x] Add support for horizontal zoom
• [x] Extend the preview to test new features
• [x] Research how drawing graphs of different types differs (if at all) and design a simple and uniform API to alter the rendering behavior adequately (the design is here: #33 )
• [x] Test thoroughly
• [ ] Explore automated testing solutions

[hgonomeg]

The https://github.com/hgonomeg/coot/tree/validation-graphs working branch is now shared with issue #33

[hgonomeg]

This can be considered done