ProjectManager: Fixed "Save As" behavior:
The project file was closed without being reloaded, causing errors if the user's dataset was stored inside the project file.
This bug was introduced in 1ebc672e81b997baa2b680c957786eed7c1ea528, in an attempt to deal with #163.
This commit fixes #163 by taking the "slow" approach for Windows: "Save As" is performed by taking a snapshot and loading it.