Improved usability of extended symbol capture

Prompt improvements and several others to make symbol capture easier and more fun.

ALSO

CNC code generation issues G00 command to PCB bottom left.

Zooming in and out using the page up and page down key has been improved. Also, holding down the shift/control key will reduce the zoom factor by 10.
Similarly panning using the arrow keys has also been improved with a similar reduction in the amount of pan by holding down the shift/control key while pressing the arrow keys.