Stable API for Settings

The request for "stable Qt Creator Plugin API" comes up regularly. We currently still need the freedom to change also core API, but for limited areas this seems in reach. This here is one of them.

A lot of plugins are using settings, organized in settings pages (currently around 100), and we have with the Aspect system in connection with the Layoutbuilder already a suitable base for reasonably concise and efficient handling of those. Both are user-extensible, but would still need some extension and polish also to handle some remaining "unusual" internal cases (e.g. tree-like data is currently not covered at all. Once this is done and the existing pages are moved to that system we can slow down the pace of change in that area.