Provide a function option to define a collector

[thorsten-reitz]

Currently, I can only create collectors in a Groovy function. Especially in conjunction with #185 it would make collectors much more powerful if it was possible to define a collector by piggy-backing it on any other function.

Using this feature, function #185 and enhancement #307 together enables use cases like these:

1. Collect IDs of all Features of a specific Feature Type and then create local references, e.g. from a Network to its members
2. Collect Geometries and then assign all of them together without requiring use of a computationally expensive Merge
3. Collect IDs of Features and group them according to a value in the source data structure so as to create references to other features in a hierarchy, e.g. for lowerLevelUnits in AdministrativeUnits

Workflow:

1. Select source and target schema element as need for the function, e.g. "Formatted String"
2. Go through the regular steps of configuring the function
3. On the "Additional Options" page, add option to collect results (checkbox) in a named collector (text input)
4. Optionally, add a source mapping anchor (selector for a field in the source data structure) to store Key-Value pairs for the collected value and a key;

Enhancement: Differentiate between the initial function creation and later function editing steps. It might make sense to have a subset of options such as "Define Collector" only available in editing, so as not to make the initial function setup process too long.