Layout nodes to grid after import of more than 5000 nodes

After importing of a large graph (200K nodes), the application was hung for several minutes (!). But working with the laid out graph was quite smooth.

Profiling revealed, that the hot spot is in OpenGL backed node rendering. My guess was that the reason is node overlaps and Z sorting. 

After implementing this fix, the same graph is 'operable' in about 30 seconds. Nothing substantial in the code, but indispensable for real use of the program.