rowmovemanager-plugin: fire slick.rowmovemanager.mousedown event.

@sergiopereiraTT : Code review please.

Issue: `handleDragInit()` stops the propagation of the `mousedown` event, this broke NTW's widget level `mousedown` handling.

Fix: Updated `handleDragInit()` to fire `slick.rowmovemanager.mousedown` event when `mousedown` propagation is stopped.