Content Search

This is a basic implementation of a content search (i.e. you can not only search in file names but also in their contents).

As for basic: 
- It will only search text (meaning .odt and the like aren't searched in properly). Though implementing this will be slow and errorprone.

- There are three free-commands that sometimes caused trouble during testing (marked in code).

- Further testing will have to be done.

I am putting this on so everyone can have a look if interested. Maybe you want to merge into master and leave the GUI controls out, so we have the feature in.