Fix for issue #6 and partial fix of issue #18: active pane management

Now correctly honour the optionActive flag to draw the pane title, and no more automatically set the active window when wxAUI_MGR_ALLOW_ACTIVE_PANE is not set and the window is redocked from floating state.

To completely fix issue #18, we would also have to show the active state when a pane is docked in a notebook and when a pane is floating.