CMS-3906 Make api.ui.time.DatePicker

- created DatePicker.ts and date-picker.less
- updated Calendar.ts
- also renamed all occurences of setText and setHtml to setHtml and moved it to Element.ts
- and extracted css from ModalDialog.ts to modal-dialog.less
Merge pull request #882 from enonic/cms-3906

CMS-3906 Make api.ui.time.DatePicker