Fix atexit function declarations for gcc and add test. Fix mangling issues for at_quick_exit and g++ headers.

Implement atexit override. Windows uses atexit is to drive invoke static destructors, and using atexit on other platforms will cause cling to crash.

Fix a segmentation violation when closing a detached TEveGedEditor, for example:
cd $(ROOTSYS)/root/tutorials/eve
root geom_atlas.C
<click on blue GLViewer [TGLSAViewer]>
<window de-attaches>
<click the window decoration to close window>
Root >
*** Break *** segmentation violation

This reverts commit f1538c4f304f470b8e24c4fc55f5724264427397.

Should fix a regression in the test suite.

Implement new naming in the relevant tutorials.
Residual fixes in the new class reorganization

This patch implenets the modifications required by the previous patch and
and makes the required internal adjustements

This patch realizes the reorganizational changes outlined in ROOT-8513.
In details:
   - Move TExecutor into core/base
   - The multi-thread Tree processor TTreeProcessor is renamed TTreeProcessorMT
   - The TTree porcessing part of TProcessExecutor is moved to a new
     class TTreeProcessorMP situated under tree/treeplayer (the interface
     is under ROOT) as TTreeProcessorMT. The methods are renamed 'Process' instead
     of ProcTree for symmetry.
   - The internal classes depending on libTree and libRIO (TPoolPlayer, TPoolProcessor)
     are also moved to tree/treeplayer and reorganized in such a way to remove code
     duplication. Their names are also changed to reflect their task.
     The new code is the new files TMPWorkerTree{.h,.cxx}.
   - The internal clas TPoolWorker is renamed TMPWorkerExecutor to reflect its task.

This interface requires the user to pass Long_t* array and lenght.
If the method contains default arguments and the size of the array is less
than all arguments we end up casting random memory.

This is a by design deficiency of the interface: we expect the user to always pass
the all argument set (including optional args).

When the write-cache required flushing to file and the length of the buffer
to be written exceed the size of the cache buffer, requiring
direct-write-to-file, the offset (seek position) was incorrectly left to
the one of the last basket in the flushed cache.

TQObject and TQConnection are used in our signal slot mechanisms. The event
listeners are modelled as a TList of TQConnections. Whenever we want to
notify these listeners (slots) we call an overload of TQObject::EmitVA. This
in turn, iterates over the listeners and calls their registered functions.

In the case of the variadic argument overload, we rely on a variadic template
to ensure we call the slot with the right set of parameters. The implementation
of this relies on calling TQConnection and we end up in a cyclic includes
TQObject->TQConnection.h->TQObject.h.

The patch splits the setting of arguments step from the actual execution. This
allows us to do the compile-time type checking. It centralizes the implemenation
around EmitVA reducing the amount of code we have to maintain.

This patch resurrects a previous overload which was turned into a templated one.
Makes it protected, and overloads it in the TQConnection class. This allows us
to remove this artifical header dependency fixing ROOT's modules builds.

Avoid duplicated effort (redo search from class name to TClass for
StreamerElement) in particular since the duplication was only partial
and contain deficiencies.

In particular TBranchElement::Unroll did not support const data member.

This was seen by CMS in a case of a class A containing an object of class B
containing an ojbect of class C which has const data member.  TBranchElement::Unroll
segfault in this case (because it did not properly search for the TClass and
assumed the search could not fail).