pcbnew: Fix dragging snap to items

Snapping while dragging depends on the current net being assigned during
dragging.  This initializes the placer to hold the current net while
performing inline dragging.  The placer is reset along with the dragger
in StopRouting().  This fixes the difference in snap between drag and
create new route.

pcbnew/router/pns_router.cpp

@@ -135,6 +135,9 @@ bool ROUTER::StartDragging( const VECTOR2I& aP, ITEM* aStartItem, int aDragMode
     if( !aStartItem || aStartItem->OfKind( ITEM::SOLID_T ) )
         return false;
 
+    m_placer.reset( new LINE_PLACER( this ) );
+    m_placer->Start( aP, aStartItem );
+
     m_dragger.reset( new DRAGGER( this ) );
     m_dragger->SetMode( aDragMode );
     m_dragger->SetWorld( m_world.get() );