Fixed docking of large toolbars.

Previously there were three problems:
1) Docking marker calculation was relative to the mouse rather than relative to the top left of the dragged toolbar.  On a big toolbar dragged from the bottom of the dragger that could be a long way off.
2) Docking markers appeared too low down.  Their position was relative to the middle of the dragged toolbar.  However as the toolbar may be resized on placement that does not make sense.
3) Large (Height()>120) toolbars could be thought to be too tall to dock in some places.  The positioning code used their current size rather than their minimum size.

src/toolbars/ToolDock.cpp

@@ -331,7 +331,7 @@ int ToolDock::PositionBar( ToolBar *t, wxPoint & pos, wxRect & rect )
             {
                // Add the new bars' dimensions to the mix
                tinfo[ct].rect = t->GetRect();
-               tinfo[ct].min = t->GetSize();
+               tinfo[ct].min = t->GetMinSize();
                tindx = ct;
                ndx--;
             }