change months structure

[Zecat]

Instead of having rows of week, months could directly contain an array of days, displayed by putting days in a container with --layout-horizontal and --layout-wrap.

[bendavis78]

Does this gain anything in performance?

[Zecat]

I suppose a little bit because it will avoid nesting 2 dom-repeat to display the months, anyway, the code will be clearer.
The '_months' structure would be changed from _months.index.weeks.index.day.index to _months.index.day.index.