Draft: Add GDK::Context container class

What does this merge request do and why?

This commit adds GDK::Context class which should be passed to other objects as collaborator. It phases out global state like GDK.root or GDK.config.

During the transition period the use of GDK::Context.current is necessary to account for hidden dependencies in GDK::Output (via GDK::Telemetry).

Contributes to #2125.

Draft?

• Add specs for GDK::Context
• Discuss the current (heh) and future approach in #2125 (comment 1973451929)

How to set up and validate locally

gdk help
gdk update

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.