lib/lp-media.c: Add support to URIs pointing to text files.

 We could use the textrender element to render text files, e.g.,

```
 gst-launch-1.0 filesrc location=TEXT-FILE
  ! capsfilter caps=text/x-raw,format=utf8 ! textrender ! imagefreeze
  ! compositor ! videoconvert ! xvimagesink
```
