Use PE display variant on any path without route overrides

Re-roll with functioning access controls.

Background: currently, PanelsEverywherePageDisplayVariantSubscriber performs an access check on the page variant by calling the parent PageVariant's resolveConditions function directly. This will always fail (for any variants with selection conditions) because the conditions returned by getSelectionConditions never undergo context mapping and thus when the execution gets to a ContextAwarePlugin there will not be a 'context' field in the configuration.

Instead, the subscriber should make use of PageVariant's own access handler, which performs context mapping.

Some style improvements and a new overridable method in the variant subscriber class.

Added $event.stopPropagation() in the variant subscriber to ensure that if a matching page is found, no further page variant changes will occur (it is Panels everywhere after all, isn't it?)

Better decision logic between overriding routes and standalone ones.

Cleaned up the variant resolution process.

Added a small helper function to get the main content from the variant, if available.

@Alumei I'm running panels:4.2 and page_manager 4.0.0-beta2 (with core:8.4.0) and it's still relevant, as Page Manager still uses the "route override" strategy for existing routes, and is still subject to Panels' default approach of rendering the page variant as the "Main Content" block *within* the original page.

@Alumei sorry, I misread your previous comment. I understand now that the 8.x-4.x branch you're referring to is for Page Manager, not Panels Everywhere. The patches I've attached here are for Panels Everywhere.

You're also right about Page Manager merged the other patched I mentioned in the issue summary, so I edited the OP to reflect that it's no longer necessary. Thanks!

@alumei gotcha, sorry for the miscommunication!

Here's a re-roll that should work.

Oops, bad upload. Here we go.