Move Functions extension point to org.dawnsci.common.widgets.gda (Cont.)

This commit covers moving KichwaFunction.java to the correct location
Implement JexlExpressionFunctionDetailPane with full sync updates and completion proposals
Remove commented out code
Fix function descriptors to not require instantiating function to query it

Added missing functionality to descriptor:
- getDescription
- getLongDescripton
- isOperator

prior to this the client of the descriptor had to call getFunction() which
would instantiate a new copy of it
Merge branch 'master' into function_fitting
Merge branch 'function_fitting'