These types were over-generalized. All use cases featured
arrays of resources attached to single parent resources. The
channel ID is sufficient to identify the child resource in the
parent, and having variadic template arguments wasn't necessary.

Futhermore we can rename these types to use the common Observer
pattern. This should make them more readable to new developers.

Also update some classes to inherit from Subject instead of
having a member Subject. This cleans up the code in a few places.

This should lead to a simpler refactor to allow dependent dirty
bits notifications in the Vulkan back-end.

In the following patch the signal_utils files will be renamed. They
are not renamed in this patch to ensure git history is preserved.

Bug: angleproject:2372
Change-Id: I17a3f2c8d92afd4bb3cba2d378c3a2e8a6d7fb11
Reviewed-on: https://chromium-review.googlesource.com/936690Reviewed-by: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>