ibusManager: Preload all ibus input sources in user configuration

Normally users switch xkb input sources and ibus input sources.
But currently the first input source only is running. It's also good
to preload all ibus engines in the logging session so that users switch
input sources quickly without the launching time of input sources.

The following is the ibus change:
https://github.com/ibus/ibus/commit/cff35929a9

https://bugzilla.gnome.org/show_bug.cgi?id=695428

js/ui/status/keyboard.js

@@ -381,6 +381,10 @@ const InputSourceManager = new Lang.Class({
 
         if (this._mruSources.length > 0)
             this._mruSources[0].activate();
+
+        // All ibus engines are preloaded here to reduce the launching time
+        // when users switch the input sources.
+        this._ibusManager.preloadEngines(Object.keys(this._ibusSources));
     },
 
     _makeEngineShortName: function(engineDesc) {