Symbol editor: minor fix do not allow the lib tree to be docked on bottom or top

of the editor frame: it cannot be displayed.

eeschema/symbol_editor/symbol_edit_frame.cpp

@@ -193,6 +193,7 @@ SYMBOL_EDIT_FRAME::SYMBOL_EDIT_FRAME( KIWAY* aKiway, wxWindow* aParent ) :
     // Columns; layers 1 - 3
     m_auimgr.AddPane( m_treePane, EDA_PANE().Palette().Name( "SymbolTree" )
                       .Left().Layer( 3 )
+                      .TopDockable( false ).BottomDockable( false )
                       .Caption( _( "Libraries" ) )
                       .MinSize( 250, -1 ).BestSize( 250, -1 ) );