Removed "isPressed" reactive property.

This was because issues using it were observed in Respondly, see
https://github.com/Respondly/respondly/commit/336584116b0aee90665b0dff8a
6ed76eeb9d3946

The problem was not at all clear, so it was decided it was safer to
remove this now to avoid propagating this kind of issue around the code
base.