Revert "Accept Result or Database object in Model constructor."

This reverts commit 361dac9eb4560eadaa0a5ba20b65e3424b75652b.

Conflicts:
	system/modules/core/library/Contao/Model.php
Revert "Pass database connections to the result."

This reverts commit 6452e127fb0e54221281df2ce8f351b33248aadd.

Conflicts:
	system/modules/core/library/Contao/Model.php
Revert "Only create a new model registry if a connection could be established"

This reverts commit 78704e55488073c03b8dbcfb4e605cb003a06a8c.
Revert "Add a registry to each database connection."

This reverts commit 4b11dbe3cf7ff1fec6aeb53599e11ae92930a421.
Make the model registry a singleton.
Safe merge related model data.