feat(wm): add visual feedback for preselection

This commit adds visual feedback in the form of a ghost tile for
preselections made by the preselect-direction command.

A container with the id "PRESELECT" will be added to the workspace, and
replaced when the next manage-able window is spawned.

A new command, cancel-preselect, has been added to remove both the
preselection index and the ghost tile if the user changes their mind.

mkdocs.yml

@@ -112,6 +112,7 @@ nav:
       - cli/focus.md
       - cli/move.md
       - cli/preselect-direction.md
+      - cli/cancel-preselect.md
       - cli/minimize.md
       - cli/close.md
       - cli/force-focus.md