[#3936] - Add overlap property globally, at report or element level

Realize a new feature from JasperReports 3.5.0

The "net.sf.jasperreports.allow.element.overlap" property can be used globally, at report or element level to specify whether element overlapping verifications are performed at report compilation time; this is useful when designing reports that are to be exported using grid exporters such as HTML and XLS, where overlapping elements do not show up;
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It would be quite significant for those who uses XLS export cause it has a severe issues with overlaped objects