[feat] Preserve backwards compatibility

[rhoerr]

Throwing this out as an idea; I haven't looked into feasibility yet:

It seems to me it should be possible to change the cache/layout handles based on each entity's settings. Even if a selectable layout exists for a given entity (which will rarely be true), it won't actually apply unless that entity has it activated in its data. That means we should be able to change the handles dynamically based on the entity data, as long as we can access the data early enough to do so.

In other words:

• Magento stock behavior is to include per-category/page/product entity layout handles always (and soon per-attributeset).
• This extension changes it to either always or never include those entity layout handles, based on system config.
• Could we instead do it case-by-case based on each individual category/page/product? If the category/page/product being loaded has a custom layout update set, or custom theme/layout active, apply its unique handle; otherwise, skip it and use the generic handle.

Theoretically, that would allow this to be used across the board without config and without breaking the custom layout feature.

It would probably be much more invasive than a single layout handle plugin.

I'm probably missing something. What is it?

(Credit to @melindash for the brainstorm)