Monor improvements in the 'highgui' module

Brief summary:
1. _modules/highgui/src/cap.cpp_ 
    * removed excessive code 
    * corrected returned values
    * corrected indentation
2. _modules/highgui/src/cap_ffmpeg.cpp_
    * constructors use member initializer lists instead of in-body assignments
3. _modules/highgui/src/precomp.hpp_
    * corrected returned values

P.S. Should be "Minor ..." in the headline. Sorry for my typo.