WIP: Allow windows to be closed from the window switchers

Implement the ability to close windows from the various non-default window switchers using the standard "close" key while the switcher is open. This works out to the "w" key for our platform.

CCBUG: 308331

This is a continuation of https://phabricator.kde.org/D16897, but implemented in a more technically correct manner.

Edited by Nate Graham

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