Adopt horde/compress PSR-4

[ralflang]

Compress is used in IMP_Application::compress - Use Horde\Compress\CompressFactory instead.
Compress is used in IMP_Contents_View::zipAttachments - Use Horde\Compress\CompressFactory instead.
Compress is used in IMP_Mbox_Import::import - Use Horde\Compress\CompressFactory instead.
Compress is used in IMP_Mime_Viewer_Tgz::render - Use Horde\Compress\Driver\Tar instead.
Compress is used in IMP_Mime_Viewer_Zip::render - Use Horde\Compress\Driver\Zip instead.

See https://github.com/horde/Compress/blob/FRAMEWORK_6_0/doc/UPGRADING.md