Inherit from QOpenGLWidget instead of QGLWidget

QGLViewer extends QOpenGLWidget instead of the deprecated QGLWidget (huge thanks to Gianluca).

• Although there are minor differences between the two classes, the switch should be mostly invisible.
• Using update() instead of updateGL() is one of them.
• The renderText methods were backported to maintain backward compatibility.
• Some overloaded constructors (using a QGLFormat or a sharedWidget) are deprecated since they have no equivalent.

Qt version suffix on Linux, minor bug fixes

• Add a -qt4 or -qt5 suffix to library file name on Linux. Fixes #18 (thanks Laurent)
• Edit the .pro of the examples to support the debug version of the library on Windows. Fixes #23 (thanks naubry)
• contribs examples no longer build by default. Relied on deprecated GL calls (Fixes #31)
• Add QMAKE_MAC_SDK to fix compilation on Mac with recent XCode versions
• Fix compilation errors on ARMv7, when qreal is defined to float (Pull request from lrineau)

Simplified build process

• Add dependency to native OpenGL drivers on Windows platform. No longer implicit since ANGLE is an alternative in Qt 5.5 (thanks Andrea and Laurent).
• The library is now always built in the QGLViewer directory. The examples' pro files are simplified and more robust.
• Add some libGlu dependency in examples that rely on it (thanks Brice).