Fix: Preserve order of selected values in multiselect TVs

Fixes issue: https://github.com/modxcms/revolution/issues/9206

before:
![bildschirmfoto 2014-06-24 um 18 50 20](https://cloud.githubusercontent.com/assets/445501/3374556/d0eba49e-fbbf-11e3-959d-746381d4c4c2.png)

after:
![bildschirmfoto 2014-06-24 um 18 50 01](https://cloud.githubusercontent.com/assets/445501/3374555/cfc91b28-fbbf-11e3-8892-b29da3e81ff0.png)