Added possibility to return tuple from filterfunc

If the returned type is a tuple, it will be interpreted as first the
title (i.e. hovertext), then the content of the infowindow. If the
returned type is a string, it will be used as both the title and
infowindow content (the old behaviour). This means that older configs
are still compatible and need no changes.

overviewer_core/data/js_src/views.js

@@ -461,7 +461,8 @@ overviewer.views.SignControlView = Backbone.View.extend({
                             'position': overviewer.util.fromWorldToLatLng(entity.x,
                                 entity.y, entity.z, overviewer.mapView.options.currentTileSet),
                             'map':      overviewer.map,
-                            'title':    jQuery.trim(entity.text), 
+                            'title':    jQuery.trim(entity.hovertext), 
+                            'content':  jQuery.trim(entity.text),
                             'icon':     iconURL,
                             'visible':  false
                     });