Support for dynamically changeable 'dragSelection'
Support for dynamically changeable 'dragSelection'

Modifies and combines the two 'mousedown' listeners for use with dragSelection.

Also adds feature to ignore the mousedown's that occur on the possibly present horizontal scrollbar (resolves issue #181)
ale-grosselle/wavesurfer.js,ffxsam/wavesurfer.js,katspaugh/wavesurfer.js,katspaugh/wavesurfer.js,agamemnus/wavesurfer.js,ffxsam/wavesurfer.js,ale-grosselle/wavesurfer.js,agamemnus/wavesurfer.js