Add public and QPA APIs for adapting existing OpenGL contexts

For now only xcb on GLX is supported. Other platforms will follow later.

Add also some missing documentation for the platform OpenGL context factory
functions.

[ChangeLog] QOpenGLContext is now able to adopt existing native contexts.

Task-number: QTBUG-37552
Change-Id: I5dd959f102df178f646b2df5989203b5dc6de376
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>
Windows: Do not link QtCore against ICU by default

Do not link against ICU on Windows, unless it is explicitly requested
('-icu' configure option). This removes the QtCore->ICU dependency
if ICU was detected in the configure environment.

So far ICU has been used in Qt Core for

- support of a larger set of codecs (805 instead of 135)
- more accurate QLocale functionality (QLocale::toUpper,
  qLocale::toLower)
- string collation

However, for all functionality there are also backends using the
Windows API/Registry (QLocale, QCollator), or built-in
codecs that are part of QtCore. Since the ICU dependency is quite heavy
(3 libs with about 25 MB) it seems sensible to not require it by default.

QtWebkit is unaffected, since it has it's own ICU check.

[ChangeLog][Windows] Changed configure defaults so that Qt5Core does not
link against ICU libraries anymore. Pass '-icu' to enable it.

Task-number: QTBUG-38259
Change-Id: I3fd3e8ac0f091f532b04945718c0e4a3cc71a087
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Include GLES 3.0 and 3.1 headers when available

In ES builds configure will now check for OpenGL ES 3.0 and 3.1
availability. This allows qopengl.h to include the correct header
and, by defining QT_OPENGL_ES_3 and QT_OPENGL_ES_3_1, the OpenGL
wrappers can provide support for MapBuffer, VAOs, etc. on GLES 3.0+
too. Right now this is not possible since the only standard way to
use an ES function specific to a given version is to use the function
directly. The extension mechanism (eglGetProcAddress and friends), that
is often used in desktop GL, is not available for such functions.

[ChangeLog][QtGui] QtGui's OpenGL headers are now automatically including
the highest available header (gl31.h, gl3.h or gl2.h) in OpenGL ES builds.

Task-number: QTBUG-38168
Change-Id: Ib857c58fe1696a9546fdd1aa143a9237e80325a5
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>