Moved /tests/demo.html to /index.html and added text stubs to check the dropdown z-index.

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rename : tests/demo.html => index.html
Moved dropdown overflow CSS to a separate 'dropdown-long' class.
Fixed the dropdown text color.
Added more stub text, discovered z-index issue.
Fixed the z-index issue by refactoring the dropdown (do not use the Bootstrap Dropdown JS part). Added some keyboard navigation.
Added monthnames option.
Reordered to day-month-year in the demo.
Added option to measure the items actual width to set the display field min-width. Renamed monthnames option to camel-case monthNames (the attribute is still lowercase data-date-monthnames).
Fixed unintended overriding of the i18n data.