Fix setting DocumentManager::currentFile when clicking into split

This broke when introducing delayed setting of the currentEditor when
the context changes. Simply move the logic from the document manager to
the editor manager, where it arguably belongs to anyway, and also set
the currentFile delayed in that case.

Task-number: QTCREATORBUG-12264
Change-Id: I67ee3f9a02e62cfa67671629c956a4290361cba8
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: David Schulz <david.schulz@digia.com>