Export Source Dialog

This pull request isn't meant to be merged, but a request for review.
Before merging I would clean up the history to reduce the number of commits, and I still have a few things to do.

But I would really like to get some thorough testing now.

Overview of changes (see also https://github.com/wbsoft/frescobaldi/issues/152, https://github.com/wbsoft/frescobaldi/issues/156 and https://github.com/wbsoft/frescobaldi/issues/159 ):

- unified exporting/formatted copying/printing to one Dialog, accessible with File->Export As
- The only additional action is File->Export with previous settings
- Greatly extend the available export options:
    - export selection/document
    - generate CSS file
    - generate document body or standalone document
    - export to clipboard/file/printer
    - style items inline, by css or by external css
    - export as plain text HTML or as rich text
    - export to HTML, PDF or ODT files (odt unfortunately only with inline styles)  
      (a wish would be PNG export (for screenshots etc.) - maybe through the poppler library?)

And any valid combinations.

Testing is needed for:

- the quality of the actual HTML output
- application logic in the dialog 
    - does it allow illegal combinations of options  
      or prevent legal ones?
    - is it possible to navigate to a dead-lock or similar problems?
    - do the results _always_ correspond to the expectations?
- Does saving the settings always work correctly?

---

What is still unfinished:

- I can't right now test with a real printer, therefore the printer settings dialog is still a sketch
- Line numbers are always styled with a css, regardless of the style setting.  
  This css has no definition (because I don't know how to inject a new TextFormat in the tokenizer mechanism)
- PDF export and printing don't respect the font selection.  
  This will be solved with the solution of the previous issue  
  (need a CSS for the `<pre>` tag)