Hide tooltip when mouse released after drag outside graph

Nicer implementation of PR #147 and tooltip now hides and reappears when release the mouse button outside the graph, after a horizontal drag.

The function to check if the event was in or outside a parent element is moved to `links.Graph.isOutside`.

By adding an event listener for the mouse-up event when the mouse goes out the graph while dragging, the tooltip can be hidden when the mouse is released (outside the graph).

The test file `js/src/graph/tests/example01_basis.html` is not really changed; only trailing spaces are removed.