This is a better fix for issue #6: active pane management
We now assume that we always track the active pane through the optionActive flag, and we just use the wxAUI_ALLOW_ACTIVE_PANE to draw (or not) the feedback of pane activation. This is conformant to the behaviour described in the documentation of the original wxAui.

This also fixes issue #21: field m_icon is now copied in wxAuiPaneInfo.