Add DateRangeSlider widget
Move jstree overrides to less/overrides.less
Add jqrangeslider to overrides.less (fix icons once more)
Temporarily allow to validate lists as tuples

This is required because we use json module for serialization
and deserialization, which just doesn't have enough information
to recover tuples from their JSON serialization format (i.e. list).
Listen to userValuesChanged on DateRangeSlider
Use DateRangeSlider in app/downloads example