Support configuration using either Qt4 or Qt5

Based on preliminary work from Steve Pieper <pieper@bwh.harvard.edu>

* vtkSlicerConfigure: Introduced Slicer_HAVE_QT5 and Slicer_HAVE_WEBKIT_SUPPORT.
  This was needed to workaround limitation of "moc" compiler preventing it
  from expanding macro like "QT_VERSION_CHECK.

* Explicitly set QT_PLUGINS_DIR and QT_BINARY_DIR variables when using Qt5

* Update extension build system to propagate Qt version

Tested with Qt 5.7 and Qt 5.9 on Ubuntu

Remaining tasks:
* Port extension manager
* Update install/packaging rules to support Qt5
* SlicerStyle (based on CommonStyle) is disabled (see Main.cxx). Application crashes on Linux.
* Fix runtime errors:
 * libpng warning: iCCP: known incorrect sRGB profile
 * QPixmap::grabWidget is deprecated, use QWidget::grab() instead

Co-authored-by: Hina Shah <hina.shah@kitware.com>
Co-authored-by: Max Smolens <max.smolens@kitware.com>

COMP adding qt5 variables for extension testing

COMP Adding Qt5 build and plugin cmake variables

From: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@26251 3bd1e089-480b-0410-8dfb-8563597acbee