HsiButton should use pressed state of ExtJS button

[chrismayer]

Currently the "HsiButton" (BasiGX.view.button.Hsi) does not use the toggle mechanism of its base class Ext.Button. Instead a custom handler function is applied which toggles a custom member variable buttonPressed. This has the effect that if the pressed state of the HsiButton is changed programatically via btn.setPressed(...) the functionality is not deactivated.

By using a toggle listener this could be fixed. The current state is available by the property pressed so no need for a custom property. This would be more intuitive and will integrate better to the whole framework.