* src/gui/editors/SongEditor.cpp
* Gradient can be toggled and color can be changed.
* Made playback line (and tail) transparent to mouse clicks
* Gradient disappears when paused/stopped; tail length depends on zoom
* Fixes bug where gradient appears when a pattern is played; style corrections
* Cleaned up code
* Rename m_zoomLevels to s_zoomLevels
* Finalising code
* Make positionLine class independent of parent zooming model
* Edit a bug fix to make it more efficient
* Rename m_x and finalise positionLine code
* Rename m_x and finalise positionLine changes
* Rename X to playHeadPos
984fd3a935