Ajja

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- Add option to switch one field or the whole form to readonly mode.

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- Use `bower-knockout-mapping` in bower package instead of serving mapping
plugin directly. (only effects bower package of `gocept.jsform`)

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- Add list widget which uses jsform to display edit form of list items.

- Remove version pin of Handlebars, since error related to Handlebars 4 is
specific to the application that uses gocept.jsform.

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- Pin version of Handlebars to 3.0.3, since switching to a new major version
should be done on purpose, rather by chance.

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- Display status messages 3 seconds instead of 1 second. This hopefully will
fix test failures in Jenkins.

- Introduced, that a save call to a widget can manipulate the source values of
another widget. This is usefull for dropdowns, where the list of available
values is dependend from the selected value of another dropdown.

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- Fix display of validation error messages.

- Make it possible to display custom HTTP error messages when `message` was
provided in the JSON response.