Load layout from configuration file

It may be easier to define some complex layouts in external files rather than build it manually in the source code.

Would need to define a layout representation (xml? json? yaml? qml?)...
We have a toml parser already to read themes, but toml doesn't seem like a good fit for this.

Using a language with an existing layout connotation (like qml or html) may look weird if we have to limit/change the actually usable elements.