Click to focus is now a global observer.
# Objective
Previously, the "click to focus" behavior was implemented in each
individual headless widget, producing redundant logic.
## Solution
The new scheme is to have a global observer which looks for pointer down
events and triggers an `AcquireFocus` event on the target. This event
bubbles until it finds an entity with `TabIndex`, and then focuses it.
## Testing
Tested the changes using the various examples that have focusable
widgets. (This will become easier to test when I add focus ring support
to the examples, but that's for another day. For now you just have to
know which keys to press.)
## Migration
This change is backwards-compatible. People who want the new behavior
will need to install the new plugin.
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