Let the cursor handle be visible regardless of selection

The cursor handle is visible on Android even when there's no selection.
Let the style decide when to show/hide the handles, but make sure to
disable the corresponding MouseArea when a handle is hidden.

Change-Id: I211c122998223ad25279bee2841c5b5e14d4c48e
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>