Add range widget

[MartijnR]

• [x] enketo-xslt - publish
• [x] enketo-transformer - publish
• [x] convert distress
• [ ] convert analog-scale? (with backwards compatibility)
• [x] check if native range can be used (instead of the slider plugin), including vertical, across browsers
• [x] add defaults in XLST or in widget (or in both?)
• [x] show steps as ticks
• [x] show min and max
• [x] default empty
• [x] ability to reset
• [x] show current value
• [x] ability to disable
• [x] reset button not clickable when vertical
• [x] Chrome ticks bug

[MartijnR]

Native is challenging but somebody has done most of the work here:

• https://css-tricks.com/sliding-nightmare-understanding-range-input/
• https://github.com/darlanrod/input-range-scss

[MartijnR]

Looks like native range inputs are out because there is no empty value. The default value is somewhere in the middle (if min < max). So if the value is empty in the model (new record, or existing record) a native range input can never show the empty value. We can however use a type="number" input as our original input and use type=range with reset ability in the widget

[MartijnR]

chrome background issue: https://medium.com/@ryanjw/buggy-edges-in-chrome-when-using-background-size-cover-dd6eb44541fc