Fix left/right margin issues with dual-panel calendars
Check whether the mousedown event belonged to a valid drop target before preventing default
Merge remote-tracking branch 'andrewnicols/dualpanel_calendar_rtl' into dev-3.x
Merge remote-tracking branch 'andrewnicols/dragdrop_prev' into dev-3.x
[Attribute-Extras]: Build.
[Calendar]: Build.
[Promise]: Build.
[DD]: Build.