Visuals + Minimum score tiles

Changed the layout to closer resemble the final mobile screen.
The timer now gets large and red when there are only 5 seconds left.
In Endless and Survival there is now an increasing chance of letters
spawning with a minimum score in the upper left. These letters cannot be
used in a word with a score of less than that. If the word doesn't score
enough, it will appear red and a message will appear informing the
player why that isn't a valid word. This also happens for words which
are too short (the minimum word length is now 3).