Make LFO controller, Bit Invader and Vibed accept custom waveforms

Changes:
*Fixes issue https://github.com/LMMS/lmms/issues/91
*Added m_ prefix to member variables of bitInvaderView
*Changed the tooltip of LfoControllerDialog

Remarks: If C++11 is allowed, the type of LfoController::m_sampleFunction should be std::function<sample_t(const float)>