[BUG #8193] Fixed typos in unit tests.
[BUG #8192] Added configuration option to be able to add an icon as opener.

- Added unit tests
- Updated demo in the websitewidgetbrowser (modified SCSS theming and added icon)
- Updated date picker implementation
[BUG #8192] Refactored implementation to use properties internally instead of exposing internals via data attributes.
[BUG #8192] Introduced 'mode' config option to be able to control which element should open the date picker. Possible values are 'input', 'icon' or 'both'. Default value is 'input'.
[BUG #8192] Added additional API documentation.
Fixed API documentation typo.