feat #1335 Range Calendar widget

This commit adds the Range Calendar widget.

It also refactors some skin properties used in the calendar,
which breaks compatibility with existing skins.

close #1335
compat Backward compatibility with calendar skin properties

This commit restores backward compatibility with calendar skin properties
after the change introduced in commit d3358a5d61519b5cd8d5d5bb10c926e372b161b4.

This commit is intended to be reverted in one of the next non backward
compatible releases.
feat #1292 Adding the ATValidateTemplates visitor in AT build and easypackage task

This commit adds the ATValidateTemplates visitor in the build and
as a configurable option in the easypackage task, so that templates
can be strictly validated during the build. This is especially useful
to find extra commas in JSON configuration objects inside templates.

This commit also adds some missing semicolons in the javascript code
generated from templates, as this is required for the ATValidateTemplates
visitor to accept templates.

close #1292
feat #1336 fullWidth property for TextField widget

Adding a new property to the textfield widget. This property allows to display the textfield as 100% width of its container.