Check whether the mousedown event belonged to a valid drop target before preventing default

Conflicts:
	src/dd/js/drag.js
Build files
Preventing users from focusing outside the modal when it's open

The user is currently able to focus on elements behind the modal, though only when they're inside iframes. This change adds code to prevent this by changing all iframes on the page to have a tabIndex value of -1. It restores the tabIndex attribute when the modal is closed.
Build files
[calendar] very_short_weekdays fallback to datatype-date-format langage data

Both shortWeekDays and weekDays will be localized at least datatype-date-format
is available in specified language. Fix #1796.
[calendar] Tweak the indent
Build files