Add "deselect all"

* New menu items in MessageList
* Show/hide menu item if anything is checked
* Clear the selected items map, redraw listview, hide button panel

Bug # 2116257
diff --git a/res/values/strings.xml b/res/values/strings.xml
index 43bf198..bf09f7c 100644
--- a/res/values/strings.xml
+++ b/res/values/strings.xml
@@ -99,6 +99,8 @@
     <!-- Menu item -->
     <string name="add_account_action">Add account</string>
     <!-- Menu item -->
+    <string name="deselect_all_action">Deselect all</string>
+    <!-- Menu item -->
     <string name="compose_action">Compose</string>
     <!-- Menu item/button name -->
     <string name="search_action">Search</string>