Implement VideoCapture::get as a constant method (bug fix #2625).

This is a port of #3503 
Since VideoCapture::get is a getter then make it a constant method. The same applies to the related methods like CvCapture::getProperty and IVideoCapture::getProperty.
It seems that `captureDC1394` is `const` here and is passed to `icvGetPropertyCAM_DC1394( CvCaptureCAM_DC1394* capture, int property_id )`. Can you check compilation with DC1394 1.x support?