Fix #1292/1294 by snapping to any edge when not hovering over a signal

1292: TimeItem instances now snap to any edge when not hovering over a
      signal, including the ruler itself. Hence, there is always an area
      where one can hover without selecting a signal, even if the entire
      view is full of signals.
      Cursors also show the desired behavior: the left edge snaps while
      the right does not. Hence, the width is kept constant. If the user
      wants to snap the right edge to a signal edge, this can be achieved
      by dragging the right edge separately.

1294: TimeItem instances now snap to any edge - before, they were snapping
      only to edges when hovering over a signal.