oxf:url-generator: don't NPE if no header value
First version of #1289 "Repeated sections"

- Form Runner
    - new fr:repeater control with repeat logic taken from fr:grid
    - fr:section inserts fr:repeater when needed
- Form Builder
    - update Section Settings to support repeat/min/max
    - switch from non-repeated to repeated section and back
    - rename controls and iterations
    - nested *-iteration holders
    - nested templates
    - some refactoring

There are some known issues which will be addressed in further commits.