Resolve links and normalize file names when opening editors again.

For the check if a certain editor is already open.
Broke with 8c2e3fd2cbaeb1178ae3991de794ee6d74c84d85.

Task-number: QTCREATORBUG-10836
Task-number: QTCREATORBUG-10945
Task-number: QTCREATORBUG-10994
Change-Id: Id1b20cd935986306f85a659f19dff52f2ee38052
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: David Schulz <david.schulz@digia.com>
Reviewed-by: Robert Loehning <robert.loehning@digia.com>