Add simple modifiers support
Simple modifiers are those that were not intended to alter characters, so Control and Alt qualify.
WIP: Depends on https://source.puri.sm/Librem5/squeekboard/merge_requests/304
WIP: Eliminate stateful locks from the history of this.
WIP: based on https://source.puri.sm/Librem5/squeekboard/merge_requests/330
Edited by Administrator