Add images for lilypond-flow (en, de)

Images displaying the relation between
- input files
- editor types
- compilation
- output file types

To be used on "Text Input"
Add images for lilypond-flow (en, de) (pdf)

Add pdf versions for use in pdf manuals
(not knowing if there's a way to reference them
accordingly in the texidoc file).
Add tex source for lilypond-flow images

The latex sources used for creating the lilypond-flow imgs.
I suggest adding source to lilypond-extra whenever possible.
This will allow translators to recreate their versions,
and it will allow editors to update the content of images.
Add script to generate lilypond-flow images

bash script that runs pdflatex on the source and then
converts the pdf to tow png files.

a translation suffix (e.g. "-de") can be passed as parameter.
Requires pdflatex (with used packages) and imagemagick
to be present, but I think this can be assumed anyway
as requirements for "build-dep lilypond"
Merge pull request #16 from uliska/lilypond-flow-image

Add lilypond-flow images