distribute object icons

- [x] distribute-horizontal-center
- [x] distribute-horizontal-gaps
- [x] distribute-horizontal-left
- [x] distribute-horizontal-right
- [ ] distribute-randomize
- [ ] distribute-remove-overlaps
- [ ] distribute-unclump
- [x] distribute-vertical-bottom
- [x] distribute-vertical-center
- [x] distribute-vertical-gaps
- [x] distribute-vertical-top

See [here](http://wiki.inkscape.org/wiki/index.php/Themable_icons) for examples

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