Express frame rate as ratio of two integer values throughout code
using struct Fraction

Add support for defining fractional frame rate from command line

- manual reformatting:
  - try to shorten lines
  - keep blocks somewhat aligned
  - remove documentation comments
- make IBC flag bool
- add some braces
- add line breaks into longer functions
- move even longer functions into .cpp file
- move static variables and functions to the end

- move VPS and related classes into VideoParameterSet.h/cpp
- move SPS and related classes into SequenceParameterSet.h/cpp
- move PPS and related classes into PictureParameterSet.h/cpp
- rename ProfileLevelTier.h/cpp to ProfileTierLevel.h/cpp and move ProfileTierLevel class there

No other changes except for minor formatting/comment cleanup

Avoid duplicating variables and functions for each reference list

- remove picture header allocations at the encoder
- use correct boundary location in reference pictures after removal of new PH

(m_layerId instead of layerIdx)

Includes:
- use of const qualifier
- fix variable naming
- use named constants