Implement a map (layer) animation control

[prushforth]

This issue is for discussion of features of an implementation of Requirement 182 for animation of map data.

It might be useful to declaratively add an animation control to the map. There are many possible designs, and we should start by considering how to make the animation accessible to accessibility technology users, conveying for example what is being animated and what the various animation "frames" mean through text equivalents. The layers should be queryable where possible, so that each frame can explain itself as required.

It would be ideal to tie this facility to a layer's use of the <map-select> <map-option> variables, although perhaps other sources of frames would be possible too.

Also, it should support time and other custom dimensions from WMS as noted in the requirement discusion, as possible.