This is a sub-issue of #1787634: [META] Decouple layouts from themes.
Layouts are only defined in code so far and there are no user interface elements to expose them yet. For blocks to be placed in layouts first we need a layout browser that shows what kind of layouts are available. The code built for this layout overview can be later reused to do block placement administration as well.
Proposed (still in the works) designs for this page looks like the following:
List of layout templates (note template terminology used for this on the user interface):
One specific layout template demonstrated:
The proposal is to add this new page in the layout module itself because this merely needs the rendered layouts to demonstrate their setup.