Implement Strike object
Use mouseover instead of mouseenter

Didn't realize the semantic difference. Mouseenter only fires when crossing the
border of the element upon which it was installed. Mouseover fires when
crossing any of that element's children, as well. Since I don't want to install
the handler on every single key, mouseover it is.
Swipe to set buzzer frequency working
Active key blue fill now follows swipe
Dragging off the piano mutes the keys
Automatically check robot connection
Merge branch 'f/swipe-striking' into develop