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
this makes LS(LEFT_CONTROL) work as if shift and control were both
pressed explicitly. Previously, the left shift would have been released
as soon as another key was pressed. The implicit behavior is useful in
case of LS(NUMBER_1) when rolling over to other keys.
Also see #361.
Peter Johanson