Sextante - Replace ParameterBoolean GUI widget with a QCheckBox instead of QComboBox

This pull request changes the widget used in ParameterBoolean for GeoAlgorithms. The widget is now a QCheckBox instead of the previously used QComboBox.

*Current ParameterBoolean  widget example (widget for 'dissolve result' parameter)*
![qcombobox_boolean](https://f.cloud.github.com/assets/732010/906907/3688d432-fcf4-11e2-8208-a2cc655a150c.png)

*New ParameterBoolean widget example (widget for 'dissolve result' parameter)*
![qcheckbox_boolean](https://f.cloud.github.com/assets/732010/906908/3686462c-fcf4-11e2-9557-33c4e3a9e69e.png)

In my opinion, this widget presents the following advantages over the previous one:
- It is better suited to the range of values (0, 1) available in the boolean type
- The GUI becomes less cluttered
- The user only needs one click in order to toggle the values, as opposed to two clicks using the QComboBox widget

Please note that this change does not affect the building of models. When creating a model it would not be possible to use a QCheckBox, because there is the possibility to add a ParameterBoolean to the model itself and then connect it to the algorithm. In this case the QComboBox is still used, as there may be several choices available.