MOCCACAL-32

Bad position for the Date/Time selector in 'Create an event' modal when scrolling
When testing I realized that this adds rules to a class which also has rules in the "xwiki-min.css".
It seems this does not work for me, because the "xwiki-min.css" is loaded later.
Maybe it is even a better idea not trying to change this in this extension (otherwise after installing MoccaCal all modal dialogs will be positioned absolute, but not without ...). I wonder if this rule can be added via javascript only to the calendar-edit dialog.   Or, if it is considered to be a general problem affecting other extensions, too, then maybe the main XWiki CSS should be updated).