WIP: Allow windows to be closed from Breeze window switcher

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

BUG: 308331 FIXED-IN: 5.20

Unlike the companion MR for all the non-default switchers (kdeplasma-addons!5), this does not quite work yet; hitting the w key while the switcher is open produces the following error instead of closing the window:

file:///home/nate/kde/usr/share/plasma/look-and-feel/org.kde.breeze.desktop/contents/windowswitcher/WindowSwitcher.qml:142: ReferenceError: StandardKey is not defined

I yet haven't had success figuring out what the problem here is. Assistance would be appreciated.