The search bar style now matches the user's theme

Custom stylesheet should be avoided, as it makes the elements look inconsistent when the user is using another theme.
So, this pull request makes the search bar gets its style from the current theme. It also add a custom class `gpaste-search` to control the padding.

![preview](https://cloud.githubusercontent.com/assets/5755892/3688280/5f3506ba-1334-11e4-82ac-be865f455c23.png)