XYPadArea: make the area keep the original DMX position internally.

Preparation for other changes, also gives higher precision
for fixture positioning.
New: Position Tool

Allows easier setting position in scene editor
Show position in XY Pads
XY Pad: Shift + arrow keys adjust fine pan/tilt, Ctrl + arrow keys increases speed (10x)
Update changelog
PositionTool: different icon, smaller size
Merge pull request #251 from janosvitok/position-tool

Position tool