rework UHK HRM macros to use $currentTime instead of hrm-timer

The `fork`ed hrm-timer macro can probably avoided by using the new `$currentTime` variable available from firmware v15.1.0, making the hrm macros completely event driven, no polling needed.

- record $currentTime on every press (or release?)
- calculate "time elapsed since last keypress or keyrelease" = "idle time"
- base decisions on amount of idle time
