Fixed an exception when adding a graphite function.

The error was that funcDef is just a string parsed from an event
like onMouseDown or something like that. This event is turned
into a function instance, and the function instance has
expected parameters and so forth. However, the number of
parameters was actually checked on the funcDef.
Merge pull request #356 from Tetha/master

Fixed an exception when adding a graphite function.