Math expression evaluator in pcbnew dialogs

[orsonmmz]

New feature: typing a math expression (optionally with distance units, e.g. 5+2*3mm) in a text input control of most pcbnew dialogs evaluates the expression when the control loses focus.

https://git.launchpad.net/kicad/commit/?id=a01d81e4b4b58490b0f88db9dbbf1090072ff3c7