Improve experimental OpenLayers viewer controls

[Demir-B]

Summary

This PR improves the controls and user interface of the experimental OpenLayers viewer used by visualize-beta.

Changes

• The experimental viewer has a new UI with more consistent styling and behaviour across the main controls.
• The overview map now consistently displays the full slide together with the current viewport.
• The layer editor UI has been improved while keeping the existing visibility, opacity and layer-ordering functionality.
• New zoom-level display alongside the existing zoom controls.
• New viewer information panel for displaying basic viewer information (WIP).
• Added a small margin around the slide extent to make panning near slide boundaries more comfortable, easy to tweak this margin.
• Improvements to the UI behaviour when loading, switching and removing slides, including the empty-viewer state.

The experimental viewer was also manually checked across slide loading/switching/removal, zooming and panning, overview-map behaviour, rotation, fullscreen, graticules and layer controls.

Screenshot of the new viewer:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.90%. Comparing base (8d13267) to head (fea567d).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1124   +/-   ##
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  Coverage    99.90%   99.90%           
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  Files           93       93           
  Lines        11911    11911           
  Branches      1562     1562           
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  Hits         11900    11900           
  Misses           5        5           
  Partials         6        6           

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