[WIP] Time compare

This PR add management for: 
 - permalink `layers_timestamp` parameter
 - list of timestamps in LayerManager (only timeEnabled layer)

Interactions between LayerManager and TimeSelector had needed  a cleaning of TimeSelector code.

Ready for a first review

Test [here](http://mf-geoadmin3.dev.bgdi.ch/dev_timestamp/)

Fix #1529  #1553 #1721 #1789 

 - [x] Missing css (size of buttons, remove transparency slider on mobile) --> will be done in another PR
 - [x] More tests with duplicate layers
 - [x] Tooltip with time for each layer #1789 
 - [x]  time parameter isn't assigned correctly to the layer 


I would rather put the popover code out of the link function (as hasLayersSameTime) and pass in the scope.