This implements several related minor but useful UX improvements for game trees:

• Large trees are initially zoomed to a reasonable size (clamped not to be too small)
• The root node is ensured to be visible when the game is first displayed
• Pressing the space bar to ensure the selected node is visible now works correctly (by fixing the event type for keypresses)
• The initial size of the frame for the game window is now adaptive relative to display size.

Closes #569.