support SelectNone ("Do Nothing") when entering a folder

REVIEW: 121116

kmail/kmmainwidget.cpp

@@ -491,6 +491,7 @@ void KMMainWidget::folderSelected( const Akonadi::Collection & col )
         case GlobalSettings::EnumActionEnterFolder::SelectOldest:
             mPreSelectionMode = MessageList::Core::PreSelectOldestCentered;
             break;
+        case GlobalSettings::EnumActionEnterFolder::SelectNone:
         default:
             mPreSelectionMode = MessageList::Core::PreSelectNone;
             break;

kmail/settings/kmail.kcfg.cmake

@@ -18,6 +18,7 @@
           <choice name="SelectLastSelected"/>
           <choice name="SelectNewest"/>
           <choice name="SelectOldest"/>
+          <choice name="SelectNone"/>
         </choices>
         <default>SelectLastSelected</default>
       </entry>

kmail/ui/miscpagemaintab.ui

@@ -126,6 +126,11 @@ p, li { white-space: pre-wrap; }
          <string>Jump to Oldest Message</string>
         </property>
        </item>
+       <item>
+        <property name="text">
+         <string>Do Nothing</string>
+        </property>
+       </item>
       </widget>
      </item>
      <item row="4" column="0">