Peter M. Groen / openbr

The primary addition is ProcessWrapperTransform, which takes a transform as a
child, and creates a worker process that runs that transform. The main point of
this is to handle cases where we interface with non-threadsafe code, and need
to isolate transforms in their own processes if we want to use multiple copies
of them.

Inter-process communication is handled using QLocalServer/QLocalSocket, the
actual worker processes are additional instance of br, which does mean this
bleeds out to the public API to an unfortunate extent.

Initial trvial tests seem promising, but generally we can expect performance
to degrade if larger amounts of data are transfered, ideally we should do
ProcessWrapper(Open+FeatureExtraction), so the data transfered to the worker
is an image name, and the data transfered from the worker is a feature vector.

Resolved conflicts:
	app/br/br.cpp
	openbr/core/bee.cpp
	openbr/plugins/output.cpp

resolved conflicts:
	app/br/br.cpp
	openbr/core/bee.cpp
	openbr/core/classify.cpp
	openbr/core/cluster.cpp
	openbr/core/eval.h
	openbr/openbr.cpp
	openbr/openbr.h
	openbr/plugins/algorithms.cpp
	openbr/plugins/independent.cpp
	openbr/plugins/output.cpp
	openbr/plugins/svm.cpp

If just the first property name argument is specified, use that for both the
gt and predicted files.

Use empty strings to indicate when the gt/predicted property names are not
supplied to facilitate the above change.

Fix argument count checks for eval regression/classification.

Change default label name from Subjet to Label (since label is a more general
term).
Use different default variable names for classification (label), regression
(regressor/regressand), and clustering (ClusterID)
Update some (far from all) transforms to accept arguments specifying their
input/output variables.
Update eval classification to optionally take target variable names as
arguments

Release a thread from the global thread pool whenever we wait for a parallel
job to finish. This means we don't waste one of the global thread pool's
threads, and can maintain full CPU utilization.

Add Show2Transform (should replace ShowTransform eventually), which displays
input images using QT, rather than going through openCV.
Additional modifications to support doing this (since qt is quite particular
about how GUI elements are created/modified). Create a QApplication instead
of a QCoreApplication (if not building BR_EMBEDDED), restructure br.cpp's main
to run its main work in a separate thread while the main thread sits in an
event loop.