Populate contao_ Symfony translations into $GLOBALS['TL_LANG']

[fritzmg]

Currently we cannot override contao/languages/ translations via Symfony translations in translations/. Also we cannot provide labels for DCA fields via Symfony translations. Our goal is to move away from the old Contao translations in $GLOBALS['TL_LANG'] and this PR provides a first step.

This PR simply populates the $GLOBALS['TL_LANG'] array with any Symfony translations that are found within the contao_* translation domains. Thus you are able to override regular Contao translations in your App like this for example:

# translations/contao_default.en.yaml
MSC:
    goBack: Back to the Future

# equivalent to:
MSC.goBack: Back to the Future

And if your App or extension provides new fields for tl_content for example, the labels can now also be provided via Symfony translations:

# translations/contao_tl_content.en.yaml
tl_content:
    myField:
        - Title of the field
        - Description of the field.
    anotherField:
        - Lorem ipsum title
        - Dolor sit description.

This makes creating translation files more comfortable when manually writing them in YAML (in case you are not using services like Transifex).

Notes:

• The new LegacyGlobalsProcess helper class extracts functionality that are both needed by the XliffFileLoader and MessageCatalogue when processing these legacy translations.
• The special handling for tl_layout is there due to the reset.css etc. option values.
• If an extension provides its labels via Symfony translations, then they can only be overwritten via Symfony translations in your App as well.

[fritzmg]

Another note: this also removes the OutOfBoundsException if "too many" nesting levels are used in the id of an XLIFF translation. I don't think this arbitrary limitation was ever necessary?

[fritzmg]

Another note: this now uses var_export (as discussed with @ausi) which also fixes some things that were inconsistent between dev and prod.

[leofeyer]

Thank you @fritzmg.