allow to clear a selection before a move is done

[christiannagel]

click on a select that's not empty, clear the selection again

[christiannagel]

The layout doesn't match anymore after the reset button was added:

The reset button also doesn't fulfill the feature to reset just a single selection (which is extremely helpful in more advanced game modes). Instead of adding a reset button, this is a way that can be done:

• clicking on a field that's not empty, resets the field, and selects it (to be filled next).
• clicking on a field that's empty just selects the field (to be filled next).

[szv]

Layout issue solved in #105