Initial commit of two preliminary plugins, videolevels and niimaqsrc
* Moved plugin_init code into main.c
* Fix batch file to use C:\gstreamer
* Improve CMake lists and modules
videolevels: add support for signed 16-bit input video
videolevels: fixes to caps handling
videolevels: fix bug with incorrect stride use in _do_levels
niimaqsrc: add copyright line to sources
niimaqsrc: correct plugin description
videolevels: add auto adjust mode and fix calculations
videolevels: implement intervals with continuous auto adjust
videolevels: fix parameter ranges
videolevels: fix auto adjust so video is immediately affected when auto is set to continuous
niimaqsrc: implement property probe interface for the interface name
build: add interface library to niimaq and instal command
niimaqsrc: add debug statements and change debug category name to niimaqsrc
untabify
niimaqsrc: minor formatting changes and doc updates
niimaqsrc: remove unused variables
niimaqsrc: Fix caps negotiation

Set caps to src pad during GstBaseSrc.start.
Use static pad templates instead of dynamically generating caps.
Force bpp to 16 even if IMAQ says 10, 12, 14 (to be fixed in the future by implementing GstBaseSrc.negotiate).