Following functions/methods have been add both to toolbar and item_proxy:
bool istoggle() const; ///< Returns true if the tool style is toggle.
bool toggle() const; ///< Gets the tool toggle state (only if tool style is toggle).
item_proxy& toggle(bool toggle_state); ///< Sets the tool toggle state (only if tool style is toggle).
std::string toggle_group() const; ///< Returns the toggle group associated with the tool (only if tool style is toggle).
item_proxy& toggle_group(const ::std::string& group); ///< Adds the tool to a toggle group (only if tool style is toggle).