Work around the fact that Contao\Database::getInstance() always creates an instance.

Contao\Database::getInstance() always creates an instance, even when no
driver is configured.
Therefore we must check that the Db is configured properly.
Cleanup uses in class BackendIntegration\Boot.
Fix issue #627 - Event dispatcher was not passed to DrawSetting::modelToLabelDefault().
Fix code style issues in Item.php
Refactor Item::isArrayEmpty() to reduce complexity.
Fix type cast code style errors (code style was changed).
Fix phpDocs.
Properly return css class.
Wrap long line.
Refactor DCA handler classes for tl_module and tl_content into base class.

Reducing copy paste occurences.
Add missing return.
Use unserialize() where appropriate.
Add unit tests for constructors of MetaModel and Base attribute.
Update MetaModel.php

Fixed check if varValue is an array
Update MetaModel.php

corrected wrong array_check
Merge pull request #633 from davidmaack/patch-3

Fixed check if varValue is an array
Store the reference of a metamodel instead of only the table name.
Merge branch 'tng' into rework-sorting
Fix issue #630 - Warnings for Content Elements and Modules.

Editing and creation of Content Elements and Modules was causing
warnings.