Add xliff export for articles

[wachterjohannes]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Fixed tickets	none
Related issues/PRs	sulu/sulu#5494
License	MIT

What's in this PR?

This PR add a command to export the articles as xliff export. In a following PR it will be possible to import the translated file again.

Why?

It should be possible to use the exported file to translate the articles with a tool or a translation agency.

Example Usage

$ bin/adminconsole sulu:article:export export.xliff en

Article Language Export
=======================

Options
Target: export.xliff
Locale: en
Format: 1.2.xliff
---------------

Continue with this options?(y/n) y
Continue!
Loading Data…
 30/30 [============================] 100%
Render Xliff…

To Do

• Add documentation page

[alexander-schranz]

@wachterjohannes currently the sulu_route.content_type.page_tree_route is not handled. It does not implement the export interface. So articles using the page_tree_route are not correctly exported or imported.

[wachterjohannes]

@alexander-schranz they have implemented it thrue the SimpleContentType which is exportable by default

[alexander-schranz]

@wachterjohannes its not tagged to be exportable as xliff can you create a PR for fixing this?

[wachterjohannes]

@alexander-schranz have done sulu/sulu#5494