Add a market button to the toolbar.

We determine the default app market activity by resolving an intent,
and fetch the icon from the activity's meta-data.
diff --git a/proguard.flags b/proguard.flags
index 607c14b..aa75f4a 100644
--- a/proguard.flags
+++ b/proguard.flags
@@ -5,6 +5,7 @@
   public void onClickSearchButton(android.view.View);
   public void onClickConfigureButton(android.view.View);
   public void onClickAllAppsButton(android.view.View);
+  public void onClickAppMarketButton(android.view.View);
 }
 
 -keep class com.android.launcher2.CellLayout {