side-by-side: Fix splitter sizes.

When checking a calendar, all existing views were updated to
the sizes of the new agenda. It should be the other way around
the new one should get the size of existing ones.

mLeftSplitter and mRightSplitter are the only splitters that
aren't destroyed each time you check a calendar, so use that
as a reference.

CCBUG: 326095

calendarviews/multiagenda/multiagendaview.cpp

@@ -576,7 +576,7 @@ void MultiAgendaView::resizeSplitters()
 
   QSplitter *lastMovedSplitter = qobject_cast<QSplitter*>( sender() );
   if ( !lastMovedSplitter ) {
-    lastMovedSplitter = d->mAgendaViews.first()->splitter();
+    lastMovedSplitter = d->mLeftSplitter;
   }
   foreach ( AgendaView *agenda, d->mAgendaViews ) {
     if ( agenda->splitter() == lastMovedSplitter ) {