Control tile layout containers via blueprint code

[jleibs]

Tile layout containers are a good first contender for expanding the blueprint APIs. Needs to represet:

• Layout type (horiz / vert / grid)
• IDs, and weights of sub-containers and panes

Some decisions to be made about what information is stored as path vs stored as components. Children could be sub-paths or batch-components.

[emilk]

We should do this first:

• Show viewport layout hierarchy in blueprint view #2140

[abey79]

• Depends on Convert ViewportLayout component to proper LayoutComponent with nested container #4285

[jleibs]

Done in:

• MVP Blueprint APIs for layout and contents #5408