Supplement to block display and select positions

This supplement is to #715 (here I have forgotten create block template), and to #717, now added "Preview" button in block list (see image below).

![Image 1](https://f.cloud.github.com/assets/628801/144596/3cdedbe8-742c-11e2-8161-dec53c4f83f2.png)

| Bug fix? | no
| New feature? | yes
| BC breaks? | no
| Deprecations? | no
| Tests pass? | yes
| Fixed tickets |
| License | 
| Doc PR |