To combine multiple sticky modifiers, the sticky keys must ignore
other (sticky) modifier keypresses.
This behavior is important for "callum-style mods", where all modifiers
are sticky mods.
Fixes#829
* Use lesser-known DT features to skip behaviors not referenced
in the user keymap
* Update the behaviors to skip code if no nodes found.
* Remove some empty config/data structs where unused in
behaviors.
