Adding equalizer support for external plugin

This modification only affects the AudioEngine by adding 3 methods :
Two public methods :
 * int equalizerBandCount() // Returns the available bands count
 * bool setEqualizerBand(int band, int value) // Set a band amplification, returns true on success

And one private :
 * void initEqualizer() // Internal init of the EQ, this adds a Phonon::Effect into the audio output

Settings and GUI must be made in an external plugin