#3 UI: Buttons are too sensitive

[Paul Wise]

Forwarding from Debian: http://bugs.debian.org/144686

In the user interface, the "buttons" (the 'main menu' button at the top of the screen, and the balls at the bottom) are too sensitive: they respond whenever the mouse is released while the pointer is over them. Since a major part of the game consists of dragging the mouse around the screen to control the camera, it's very easy to accidentally release the mouse in the wrong place and have the menu pop up or (very annoyingly) the camera recenter on another ball.

The buttons should only respond to mouse-up events when a mouse-down was previously received *on the button* (in which case the camera shouldn't respond)