Added support to PyQt5 API in the qt compatibility module

Now PyQt5 is the reference API and the default Qt binding.
Modules that are specific of the PyQt5 API are not available via
the qt compatibility module but they have to be imported directry
from the PyQt5 package.
Port exectools to PyQt5 API
Remove all explicit references to Qt4 in exectools
Update module header lines in exectools
Add support to Qt5
Never use the deprecated QMatrix class (not available in PyQt5)
Fix QItemSelection*
Fix clipboard usage (PyQt4 --> PyQt5)
Fix QTextDocument usage (PyQt4 --> PyQt5)
Fix QPrinter usage (PyQt4 --> PyQt5)
Fix QTextCursor usage (PyQt4 --> PyQt5)
Fix QTextTableFormat usage (PyQt4 --> PyQt5)
Fix QTextFrameFormat usage (PyQt4 --> PyQt5)
Fix QTextBlockFormat usage (PyQt4 --> PyQt5)
Fix QTextCharFormat usage (PyQt4 --> PyQt5)
Fix QFont usage (PyQt4 --> PyQt5)
Fix QPrintDialog usage (PyQt4 --> PyQt5)
Rename modules: qt4 --> qt
Fix wheel handling
Move test files from gdalbackend to the tests folder