Add `fallback` to shim

Tipsy also supported the `fallback` element which will now not work anymore. To enhance compatibility we shall also implement it in the shim.

Fixes https://github.com/owncloud/core/issues/17870

core/js/js.js

@@ -1888,6 +1888,8 @@ jQuery.fn.tipsy = function(argument) {
 		}
 		if(argument.title) {
 			options.title = argument.title;
+		} else if(argument.fallback) {
+			options.title = argument.fallback;
 		}
 		// destroy old tooltip in case the title has changed
 		jQuery.fn.tooltip.call(this, 'destroy');