feat(formFields): Add cancel and esc handling and CTRL+Z to dialog.

- ModelerDialog contains the whole implementation for CRTL+Z, cancel and
  esc in dialogs
- add validation for mandatory fields
- make help text for mapping tables configurable

- "Form Fields" tab is now activated in the code base

related to #CAM-1398
Merge branch 'master' of github.com:camunda/camunda-modeler