mus Window Server: implement event capture

This CL implements an explicit capture API that will be used for popups.

This is a split of the review: https://codereview.chromium.org/1605773004/
Where this review encompasses only the Window Server functionality. The client lib code is just stubs for compilation. Full implementation of the client will follow in a separate review.

TEST=EventDispatcherTest.SetExplicitCapture, EventDispatcherTest.ExplicitCaptureOverridesImplicitCapture, EventDispatcherTest.CaptureUpdatesActivePointerTargets, EventDispatcherTest.UpdatingCaptureStopsObservingPreviousCaputre, EventDispatcherTest.DestroyingCaptureWindowRemovesExplicitCapture
BUG=533161

Review URL: https://codereview.chromium.org/1677513002

Cr-Commit-Position: refs/heads/master@{#374824}