New input features: extend segments, perpendicular, snap to shapes

The tools of the ancients have been revealed to us - draw lines, arcs and circles as they were originally intended to be drawn.

New features:

• Overlay with controls and help info (toggle with F1)
• Extending segments
• Drawing segments at a perpendicular
• Snapping to shapes
• Drawing the point equivalent of a shape
• New file shortcut
• Better opening of files
• Cursor change based on current action
• Store divider lengths as letters
• Divide/multiply current length by integers with the number keys
• Swap to previous length with Tab
• Better drawing previews

The input method has changed to allow for all the new input methods (hit F1 to see controls). This will probably change again and be consolidated once (if?) editing shapes and points is added. So don't get too attached!