Design and implement responsive ensemble spec

[evshi]

Is your feature request related to a problem? Please describe.
Currently responsive design in ensemble is a mix of conditionals and expressions using device.width/height. This can be very tedious when there are many responsive elements.

Describe the solution you'd like
Design a YAML directive that allows users to specify responsive behavior.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Something like CSS media queries would be ideal

[evshi]

@vusters @amin-nas let me know if you have an existing thoughts on this.