Add template event at the end of the file

Currently build-in JS scripts (forum_fn.js, ajax.js and plupload stuff) are being included using INCLUDEJS which makes them to be loaded in the $SCRIPTS template var.
As scripts are loaded in $SCRIPTS randomly, ext authors can't control this order to put their JS scripts after the build-in ones to be able to communicate to them when the page is loading (f.e. to plupload), like redefine some functions, modify vars etc.
As such, we can add the lowermost template event for ext authors to be able to handle this case in some way.