#48 Robust formula editor

[Frank W. Zammetti]

By "robust" I just mean more than a textbox :) Things it might include:

* A dropdown of functions supported by the selected language with the ability to click one and have it inserted at the current cursor position

* Simple syntax checking (I suspect we could insert test values for various tokens and run the formula and see what errors occur)

* The ability to save formulas in preferences so you build up something of a snippet library across reports