feat(wm): stack out of sight on a parking desktop

A container has one tile, so the windows of a stack that are not in front have
to be out of sight. KWin has nothing a script can set to hide a window except a
virtual desktop nobody looks at, which the first spike measured to cost no focus
change where minimizing a window cost two. Backend::hide takes a window out of
sight and Backend::restore brings it back; the KWin backend parks the window on
a desktop named komorebi-park, which the table keeps out of the workspaces, and
remembers where each hidden window came from so that it is still on its
workspace.

KWin saves a desktop in kwinrc as soon as it is made, so komorebi removes the
parking desktop when it stops, after the windows are back, and the first
snapshot of the next run finds one a killed run left, sends its windows home and
removes it. The windows komorebi hid are recorded, so that a minimize the
compositor reports for one is not mistaken for the user's.

komorebic stack, unstack, stack-all and unstack-all stack and unstack windows.
The window container behavior and the float override decide whether a window
that opens joins the focused container or floats, for every workspace or for
one. vm-stacks stacks windows in a headless KWin, shows a workspace and goes
back, stops, and kills the daemon with a window parked.

docs/cli/unstack.md

@@ -0,0 +1,10 @@
+```text
+Unstack the focused window
+
+Usage: unstack
+
+Options:
+  -h, --help
+          Print help
+
+```