Improved file buffering in CArchive.

CArchive already did some file buffering, but only on writes. Added the
equivalent code for reads. Also improved the write buffer case so that it
only ever issues sector-aligned writes (the read code does this from the
start). Shuffled various bits of code into the header file to squeeze a bit
more performance out of it.
[application] - remove m_bInBackground and its setter/getter and use
SetRenderGUI instead
[json] - fix jsonbuilder was not found when added to "PATH" from outside. Add the same approach like done in codegenerator.mk - fixes json generation when compiling from xcode
[airtunes] - phase out support for libshairport - its superseeded by libshairplay long enough on all platforms
Merge pull request #4694 from Memphiz/fixjsongeneration

[json] - fix jsonbuilder was not found when added to "PATH" from outside...
Merge pull request #4234 from bavison/carchive_buffering

Improved file buffering in CArchive.
Merge pull request #4697 from Memphiz/phaseoutshairport

[airtunes] - phase out support for libshairport - its superseeded by lib...
Merge pull request #4693 from Memphiz/remove_background

[application] - remove m_bInBackground and its setter/getter and use
[rpi] Include ntsc frequencies in list of supported resolutions
[omxplayer] Allow a framerate callback from GPU to trigger a hdmi mode change
[omxplayer] Request to be notified about framerate changes