fix for dragging last tab from a window and change to use @rpath

When dragging the last tab from a window the alpha for the drag window was being set to 0.0 thus rendering the drag image invisible. The tab bar button was not shown when hovering over the destination tab bar and no mini or alpha window was seen while dragging.

I also updated the INSTALL_PATH to use @rpath which is generally preferred for OS X 10.5 and above.