Allow for custom capture logic

Hallo,

In the following branch:
using a form config with the optional Setting captureMode: custom 
```
  GraveWork:
    label: GraveJobs
    layer: Grab
    type: custom
    captureMode: custom
    widgets: []
``` 
Triggers the use of the map tool CopyTool to capture the Feature selected by the Capture Tool and provide it to dist/Projects/X/Y/formXY.py:XYform(FeatureForm), where it can be used to populate elements in form.ui or to save data.  The saving of data when captureMode: = custom and the property featureform.customSave = True is left exclusively to XYform.py.

I realize that customSave will be implemented as a fully integrated part of the roam design.  This pull request is made as an example of how I have implemented the functionality, in the hopes that it will serve as a useful example.  2 sample Projects are provided for testing purposes.  If I can be of help in getting this functionality implemented, let me know.