Make QQuickRenderControl public

QQuickRenderControl allows rendering Qt Quick 2 scenes into framebuffer
objects which can then be used in arbitrary ways in Qt-based or 3rd party
OpenGL renderers.

[ChangeLog][QtQuick] Introduced QQuickRenderControl as a public API.

Task-number: QTBUG-37944
Change-Id: I84262243b261b35cefdf67ec6bba8127a0f29275
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>