Allow multiple defaults and "Panel choice"

Depends on #2664682: Implement admin UI for editing Panelizer defaults getting implemented first!

This issue is to allow the user to create multiple Panelizer defaults for a given entity type, bundle and view mode. This is already supported at the API level: Panelizer::getDefaultPanelsDisplays() returns an array of named defaults, where the one we're currently used is named 'default'. We also store the name of the default used in PanelizerFieldType. We really just need to allow the user to create other defaults, and then choose to use them.

So, we need to:

• Add the admin UIs to create multiple named defautls
• Add support to choose the default in PanelizerWidegt
• Add support in our Panels IPE integration code