Deactivate all unchecked checkboxes if current dialog has only one selected element and there are already two multiple dimensions

- Rename "selectedElementCount" and "elementCount" to "__cv_selectedElementCount" and "__cv_elementCount"
- Partly improvements for #135
Deactivate all checkboxes if dialog is related to an one element dimension and there are still two multiple dimensions

- if you uncheck the last checked element, you can check one of them again
- partly improvement for #135
Deactivate select/deselect buttons in case that there are checkboxes deactivated