Switch from EventSubscribers to core-style ContextProviders

Working on this.

Unfortunately we have to duplicate ALL of LazyContextRepository just to pass along a parameter.

Didn't have enough test coverage, static contexts didn't work.

+++ b/src/Context/PageContextRepository.php
@@ -0,0 +1,165 @@
+class PageContextRepository extends LazyContextRepository implements PageContextRepositoryInterface {

This is the worst class I've ever written.

+++ b/page_manager.services.yml
@@ -1,14 +1,20 @@
+  # Missing argument is purposeful, see \Drupal\page_manager\ContextProvidersPass.
+  page_manager.context_repository:
+    class: Drupal\page_manager\Context\PageContextRepository
+    arguments: ['@service_container']
+

Trivial to the scope of this patch but I personally prefer providing the empty array and documenting where its populated rather then leaving it missing. Makes things more clear when reading the services file what you'd expect on the other side.

arguments: ['@service_container', []]

The compiler pass can also pull in the argument and merge it so it would work if someone where to write their own compiler pass and wanted to run first for what ever reason.

Edit: remove dreditor weirdness.

Will try to review this asap. I think it's going in the same direction as what I wrote about in #2511568: Create "context stack" service where available contexts can be registered. I need to look at this more closely, but some of the things I'm thinking about there that this probably doesn't cover yet:

* Do it in ctools, based on a generic interface
* Support multiple objects like a page manage page with their own context, by using that object as static cache instead of a property. Page manager itself already has that I think, with the page and the variant, although one is a superset of the other.
* Also take care of static contexts and relationships, in some way

Patch seriously outdated after contexts moved to ctools

Patch needs work cos accepted #2633052: Page variant by current language