Improve collection fields

[miguelgrc]

• Tab: should we allow nested tab fields?
• Layer: should work both as an array and as an object. When it's an object, a button can be added to open it, instead of just clicking anywhere on the item.
• Accordion: should work both as an array and as an object. Should have the title template (stringify) functionality.
• Add as well a "compact mode" for arrays, which when enabled basically applies a CSS rule to reduce the paddings for array items (in principle for Layers and Accordions)