Provide support to properly zoom / drag the heatmap

[pverscha]

Right now, the heatmap is a static visualization. It's not possible to zoom / drag the visualization if required, making it hard to interpret large heatmaps. This functionality should be provided by the heatmap in order to provide a much better user experience.

[pverscha]

A lot of progress has already been made in the branch feature/zoomable-heatmap. This video shows a summary of what the heatmap is currently capable of:

Note, that in order to improve the heatmap's performance, I also decided to switch from an SVG-based to a canvas-based implementation. This migration is mostly done, with only the clustering that still needs to be migrated.

Highlighting of the currently selected square has also been implemented, together with automatic label truncation and optimization.