Clean up the main ShootOFF window (several minor issues)

[phrack]

• If the canvas is a bit bigger than the feed from the webcam, hidden targets can be seen at the bottom and right of the canvas. On most systems the cavas has a few extra pixels on the right and bottom.
• The shot timer's horizontal scrollbar is not all the way to the bottom. This obscures the view of the bottom item when there are many shots recorded.
• The clear shots button and clear shots target should be renamed to "Reset" to more clearly describe the function they now perform. References to these need to be updated in the wiki including pictures.