If a key is not set, a PHP Notice is triggered. While these are
currently suppressed, this is still a code smell.

There was a bug that the empty string was considered to be invalid if
also a max or min value was defined for a config field of type
numericopt.

This makes sure the provided extension ID is a valid id before
initializing it in the extension manager.

The internal lang and localized variables of a plugin are no longer
accessible from the outside. This adds a proper setter and also fixes
the type hinting in the config plugin.

this fixes another problem with our tests and phpunit 6.1

This should fix some tests on PHP 7.1. The work is far from done, we
still have a lot of variables without proper accessibility defined.