Add source code export commands

- export colored (styled) HTML (body only)
  Do the same as the existing Export Coloured HTML but only export
  the content of the html file within but not including the <body></body> tag.
  The result can be used to be inserted in HTML documents that already
  have a link to a CSS file with Frescobaldi's styles

- copy styled HTML
  Copy the selection to clipboard as HTML source code
  This references but doesn't include stylesheets
  (to be used to paste in places that have an appropriate stylesheet)

- export CSS file
  Assemble a CSS file matching the current knowledge
  of the highlighter.
  To be used for example in a website design template

Resulting feature requests:

- New interface to Export Source:
  Dialog where both options can be set:
  - inline/css (default css)
  - body-only/complete-document (default complete)
  Filename defaults to %.html,
  optionally the FileSave Dialog can be called
- Include line numbers in export and copy (optionally)
Refactor copy source commands

Factor out common code, can now easier be extended by new commands
Add "copy as (inline) styled HTML code"

Copies selection as html code styled with inline styles
that can be pasted in any html page without the need
for a style sheet.

Renamed the three 'copy' commands to be more consistent (I hope)
Clean up exportCSS

Remove some unnecessary (copied) code
Improve interface:
- suggest "lilypond.css" as filename
- use current dir as default or
- user's home dir if current file isn't saved
Add "export source as (inline) styled HTML (body only)"

... and refactor the functions out a little bit

It would be easy to add inline styled export for the whole document,
but I don't think that makes any sense.

When there will be a new dialog interface for this
we won't need the individual entry functions anymore.
Instead the flags will be set according to the Dialog's results