Merge tag '4.4.1' into develop

Version 4.4.1
Use PSR-4 autoloader.
Require composer-plugin instead of composer-installer.
Require contao/core >=3.2,<3.4-dev
Require bit3/contao-assetic ~1.0
Require bit3/contao-meta-palettes ~1.0
Require contao-community-alliance/event-dispatcher ~1.0
Require contao-community-alliance/dependency-container ~1.0
Remove deprecated source.
Remove unnecessary LGPL.txt
Move the proxy.php into public assets directory.
Extract the insert tag filter into its own repository.
Refactore using events and a lot of cleanup.
Merge branch 'release/5.0-alpha1'