Allow skins to register their own button and disable compact

Example usage:
Iabd4688c6081b4de391b9655b92a16f3a414e018

We will skip loading the compact link if we don't need
to compact the languages.

Note special handling is known for Timeless which stops
propagation of events. This will be fixed in Timeless at a
later date.

Bug: T264824
Change-Id: I0518ecdf402ebf5eb6bad2c430f6462322c0d8e1

extension.json

@@ -159,8 +159,7 @@
 		},
 		"ext.uls.compactlinks": {
 			"packageFiles": [
-				"js/ext.uls.compactlinks.js",
-				"js/ext.uls.launch.js"
+				"js/ext.uls.compactlinks.js"
 			],
 			"styles": "css/ext.uls.compactlinks.less",
 			"dependencies": [
@@ -224,7 +223,10 @@
 		},
 		"ext.uls.interface": {
 			"targets": [ "desktop", "mobile" ],
-			"scripts": "js/ext.uls.interface.js",
+			"packageFiles": [
+				"js/ext.uls.interface.js",
+				"js/ext.uls.launch.js"
+			],
 			"styles": "css/ext.uls.interface.less",
 			"dependencies": [
 				"mediawiki.jqueryMsg",