Show more blockpossitions in blocks modify

This is small UI tweak in Blocks modify screen.

Current status: Blockpossitions select is fixed to 4 rows in size. Even default theme Andreas08 have 7 blockpositions, Multiselect is difficult, and not visible in many cases.

Proposed changes in this PR: Now select and multiselect is easy and visible.
![Image 2](https://f.cloud.github.com/assets/628801/134939/243c443e-7104-11e2-99a0-009de441a4b0.png)

| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | 
| Doc PR        |