Improvements to drag and drop, iOS Safari

Minor quality of life improvements in Huewords mainly for the phone / tablet players.

- Highlight the current target square of a drag and drop operation with an animation

- Don't allow drag and drop of letters into a group of the wrong size

- During drag and drop, visually de-emphasize the squares that the letter can't be placed in

- Fix issue with some taps made in quick sequence being missed on iOS Safari