Rename new QOpenGLContext APIs

isES() becomes isOpenGLES(). The library type enums are changed
DesktopGL -> LibGL and GLES2 -> LibGLES. This removes the now
unnecessary version number, the confusing "desktop" term and provides
better readability.

The old function/values are kept until the related qtdeclarative
changes are integrated.

Task-number: QTBUG-38564
Change-Id: Ibb0a1209985f1ce4bb9451f9b7b093c2b68a6505
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>