ExtensionManager: Switch on by default

But keep experimental.

It is currently less functional than the old plugin dialog, it
misses e.g. dependency tracking when disabling plugins and the
design misses a way to restart Creator in the common case where
this is necessary.

Change-Id: Ib6c5aba762426510fc174c99ddc486d31d240484
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
(cherry picked from commit c57d70e242)
Reviewed-by: hjk <hjk@qt.io>

src/plugins/extensionmanager/ExtensionManager.json.in

@@ -15,6 +15,7 @@
     "Category" : "Core",
     "Description" : "Extension Manager",
     "Experimental": true,
+    "DisabledByDefault": false,
     "Url" : "https://www.qt.io",
     ${IDE_PLUGIN_DEPENDENCIES}
 }