Disabling/enabling EGL functions changes the DEF files, stub EGL implementation needed

When OpenVG is enabled, it will via the EGL functions create additional exports, which in turn change the DEF files on Symbian. There are a number of solutions to this, the simplest one for now being:

1. create stub functions for the EGL functions
2. build either the full or the stub functions depending on the selected graphics system
3. always link the stub/full EGL functions to the QtGui component
4. refreeze the QtGui DEF files with the EGL exports