Enhances package installation plugins

Enhances package installation plugins

Changes:

* Adds `DatabaseObjectDecorator::getDatabaseTableAlias()` (needed later)
* Removes obsolete `PageLocationPackageInstallationPlugin`
* `AbstractXMLPackageInstallationPlugin::$tableName` is automatically set if it is empty and `AbstractXMLPackageInstallationPlugin::$className` is set.
* `AbstractXMLPackageInstallationPlugin::$tagName ` is automatically set if it is empty and `AbstractXMLPackageInstallationPlugin::$tableName` is set/has been automatically set.
* Updates/Adds some comments
* Removes obsolete `ACPTemplatePackageInstallationPlugin::$tagName`
* Fixes #769