[UX] Flexible layout templates: Provide a selectable list of "presets"

When you create a new template in the layout template builder, you start off with a specific default set of rows and regions (3 rows, with each one having a single 100%-width region in it):

It would be useful to present options to copy the regions/rows from existing layouts. Here's a rough mockup of how that could look like:

There may be many such presets to chose from, so they could be hidden in a fieldset instead; or the "Copy regions from existing preset" thing could be a link that opens a dialog.

If #4483 is rejected, then we could add these presets in the "Add template" step (where we ask for the name and description of the template):

Anyway, the purpose of this is to allow people to quickly get to where they need to, by allowing them to select a preset that is closer to the regions they need. They could then tweak it to get to 100%, instead of having to start from scratch.