[Discover] Add Visualize button also for multifields

[kertal]

Currently, when there's a multi field displayed in Kibana's sidebar, it doesn't show a visualize button.

Field with Visualize button

Multi field without Visualize button

It would be neat if, if there are multi fields available, and one of them would be visualize-able, the button was displayed.
If there were more than one suitable fields, the first one should be used to create the visualization.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)