R interface: support VF2 functions with callbacks

[gaborcsardi]

From @gaborcsardi on May 4, 2013 18:59

Should not be hard, the same way as for ARPACK and BFS/DFS.

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Imported from Launchpad using lp2gh.

• date created: 2009-04-16T14:49:15Z
• owner: gabor.csardi
• the launchpad url was https://bugs.launchpad.net/bugs/362353

Copied from original issue: igraph/igraph#381

[gaborcsardi]

Callbacks are evil, one cannot call any igraph function from them, currently. So I am closing this for now.