impr(form): provide a directive to show a form

This commit

- removes two behaviors to show a form
- implements a single directive to show a form which will be used for a
  task and to start a process
- adds the behavior that whenever a task (or process definition start
  event) has not a dedicated form (embedded or external) a generic form
  will be shown. This generic form offers to enter several variables.
- removes the typeahead to search for process definitions
- etc.

related to CAM-2637
impr(filter): prevent propagation of click event to edit or delete
fix(variable): change the modal close button appearance