Fix drag-zoom and pan for non-linear scales

[kurkle]

Pan and drag-zoom were not working properly when the scale is of non-linear numeric type.

What is changed:

• drag-zoom is based on getValueForPixel for all scales. I.e. the values at the rectangle corners are resolved and used as new boundaries
• allow range to change when panning non-linear scale
• Many new samples + some re-organization

Some issues left:

• Pan on timeseries scale does not really work (chart.js issue)
• Pan non-linear scale against a limit results to unwanted zooming

Resolves #180