MOVABLE="TRUE" reuses existing instance when changing docking position --
cf. http://www.jedit.org/api/org/gjt/sp/jedit/gui/DockableWindowManager.html
<?xml version="1.0"?>
<!DOCTYPE DOCKABLES SYSTEM "dockables.dtd">
<DOCKABLES>
<DOCKABLE NAME="Isabelle_output" MOVABLE="TRUE">
new isabelle.jedit.OutputDockable(view, position);
</DOCKABLE>
<DOCKABLE NAME="Isabelle_state" MOVABLE="TRUE">
new isabelle.jedit.StateViewDockable(view, position);
</DOCKABLE>
<DOCKABLE NAME="Isabelle_scroller" MOVABLE="TRUE">
new isabelle.jedit.ScrollerDockable(view, position);
</DOCKABLE>
</DOCKABLES>