Accessibility for touch controls!

Hey everyone!
More slight tweaks for today.

• A new erase button has been added to the element select panel to help players on a touch screen. Simply click it to enter an erasing mode, where tapping tiles on the grid will delete them.
• The element select panel also now displays icons to make the different stats of each element more obvious. The left column is liquid, the right column is solids. The top row is cold elements, the bottom row is hot.

Enjoy!