Add posibillity to set custom color to specific files (Issue: #160)

- Can be enabled/disabled in the 'view' menu
- Custom colors can be set in the files 'properties' dialog
- It is possible to pick a custom background color and a custom text color per file
- Usage of gvfs metadata to store the file specific color information
- Introduces a custom text renderer to get the job done

MR: !226