Removed quickElement from calendar.js

The function quickElement in calendar.js is a duplicate of the
quickElement function in core.js. Refs #4120.
Moved cancelEventPropagation into core.js

The cancelEventPropagation function is a general utility function that
can be reused by other widgets. Refs #4120.
Moved removeChildren to core.js

The function removeChildren is a general utility that other functions
might want to use.
Fixes #4120. Thanks arvin for the initial patch.