Replace visRatio and layout props with aspect prop

[axelboc]

Fix #1275

⚠️ Significant breaking changes

• [VisCanvas] Prop visRatio is replaced with prop aspect, which follows matplotlib:
  • 'auto' is equivalent to no visRatio (i.e. the heatmap's 'fill' layout);
  • 'equal' is equivalent to computing visRatio from the axis domains to obtain an orthonormal basis (i.e. the heatmap's cover layout) -- so now, this ratio computation is performed internally by VisCanvas ;
  • and if aspect is a number, then it is used as the ratio directly (this is not used by the viewer; it's just for flexibility of the lib and consistency with matplotlib).
• [HeatmapVis, RgbVis] Prop layout is replaced with prop aspect, which is given sane defaults ('equal') and forwarded to VisCanvas.

I've also replaced the layout property in the config stores and toolbars with a keepRatio property, since it is all we care about in the viewer.

[loichuder]

I like the change !