Nux doesn't give the mouse button used while we are dragging

[Didier Roche-Tolomelli]

see https://code.launchpad.net/~didrocks/unity/fix-709179-709119/+merge/47802

With a NUX_IMPLEMENT_OBJECT_TYPE (Launcher),

The connected signal is OnMouseDrag.connect (sigc::mem_fun (this, &Launcher::RecvMouseDrag));
With:
void Launcher::RecvMouseDrag(int x, int y, int dx, int dy, unsigned long button_flags, unsigned long key_flags)

However, whatever mouse button is used to perform the drag, button_flags keeps being 0, so we need to make some hooks in MoUseDown/MouseUp to know which button has been pressed for the drag.