[#5840] - Creating new sheets for subreports when exporting to xls/xlsx

Tested on 4.5.1, made for 4.5.1
We wanted to be able to have an easy/intuitive way to separate subreports into separate sheets.
Currently you can apply the break before/after properties to elements and to groups, but not directly to a subreport. Basically it's a not a problem as you can find a way to break around a subreport, but we wanted an easier way.

This adds the property net.sf.jasperreports.export.xls.break.before/after.subreport, which defaults to false and is applied at the report level, and allows you to add .break.before/after.row to a subreport to override the report level setting on a by subreport basis.
If you export with pagination, each subreport is one sheet with several pages (though I suppose if you had one sheet per page that might be interesting).
Finally, the property only applies to subreports at the current level. If you put a subreport in a subreport and enable breaking, the subreport in the subreport won't break into another sheet unless you set the property in the subreport's jrxml.

Maybe it's a bad idea since it requires a patch at the filling stage, but we found it useful so may as share.