bugfix: goto -> select all

Whenever you open the Goto prompt, it should select all the text, so you
can either hit Enter to go to the same location again, or just start
typing to go to a new location.
Prep work for image interpretation

The next step will be recognizing image/palette data. Here, we introduce
the LZ utility functions from the testing project and we add a new
Element Factory to interpret the image data.