WindowSwitcher: correctly handle multiple keybind modifiers #6050

Job	Run time
fedora	1m 26s
open-suse-leap	1m 43s
lint	11s
build-and-test (stable)	1m 19s
build-and-test (unstable)	1m 19s
build-and-test (development-target)	1m 25s
	7m 23s

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