Bots now have configurable skill levels

The bots now have selectable skill levels (Beginner, Intermediate, Advanced). If you can beat the Advanced bots I'll be really impressed :)

Also made several UI tweaks and quality-of-life enhancements. The buttons are bigger, the cards should be easier to distinguish highland ladd from highland lass (important when playing to trading piles). You can now "swipe" through the tutorial pages.

Eliminated drag-and-drop since I find myself always double-clicking to play a card. Give UI hints for things like that.