Allow TSVB legend sorting by value

[AlonaNadler]

Kibana version:6.1.1

Elasticsearch version:6.1.1

Browser version: Chrome

Original install method (e.g. download page, yum, from source, etc.):tar.gz

Description of the problem including expected versus actual behavior:
If there are multiple series defined in the TSVB, the way we display the legend today is ordered by series first and then values within that series. Which means that it will always display the first series in a block first. So even if the 2nd series has the highest values, users will have to scroll down the legend to the bottom to get to these highest value

Describe the feature:
Ability to define the legend order by value (highest or lowest) so when a user hovers on areas in the chart the legend display will show the relevant series.

[alexfrancoeur]

cc @simianhacker

[sulemanof]

Closed in favor of #46466