DPL: move GUI to a plugin

[ktf]

This moves the whole GUI implementation to an on demand
plugin which will be loaded only if we are not in batch mode.

This will both save memory (30MB PSS, in my tests) and speedup startup
when running in batch mode.

[ktf]

@davidrohr do you understand why we get:

[ 81%] Generating GPUReconstructionOCL2Code.src
In file included from /mnt/mesos/sandbox/sandbox/o2-fullci/sw/SOURCES/O2/5099/0/GPU/GPUTracking/Base/opencl-common/GPUReconstructionOCL.cl:72:
In file included from /mnt/mesos/sandbox/sandbox/o2-fullci/sw/SOURCES/O2/5099/0/GPU/GPUTracking/Base/GPUReconstructionIncludesDevice.h:98:
/mnt/mesos/sandbox/sandbox/o2-fullci/sw/SOURCES/O2/5099/0/GPU/GPUTracking/ITS/GPUITSFitterKernels.cxx:25:10: fatal error: 'cstdio' file not found
#include <cstdio>
         ^~~~~~~~
1 error generated.

is CA_DEBUG actually defined in the fullCI?

[ktf]

clang-format actually wrong. fullO2 fails now but I cannot find any error in the log.