Add context adoption support for WGL

This is trickier than the GLX and EGL implementations due to the
way pixel formats, windows and contexts work. Apart from some
restrictions, it should be fully functional nonetheless.

Add also some proper documentation.

Change-Id: Ia6e3eb1ab2701e439b8621b9092c2b0934ff2151
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>
Fix corruption with QQuickWidget and QOpenGLWidget on ANGLE

This only ocurred in -opengl dynamic builds on Windows, when using
ANGLE. The !QT_OPENGL_ES_2 code path did not correctly check for ES.

We also need to avoid using REPEAT, just to be safe on ES2 implementations
(embedded devices typically) that will not do repeat on non-power-of-two textures.

Task-number: QTBUG-39638
Change-Id: I2eeab6627c17255373193c0a0fec8923f711e28c
Reviewed-by: Andrew Knight <andrew.knight@digia.com>
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>
Accessibility: Improve handling current value

Value interfaces on OS X and iOS can be strings, so can the value
property of MSAA. Before we'd always only send doubles, instead
change it to use strings as well.

Change-Id: I1b4410c68238ba7a69a5507d87c251f2ac61c568
Reviewed-by: Caroline Chao <caroline.chao@digia.com>