#14 'Save as' fails without error message

[Thijmen de Gooijer]

When 'save as' is used from the file menu, the function fails if a subnet editor was active on the workspace when the function was called. It only works correctly if 'save as' is used from the upper-level diagram editor.

Steps to reproduce:
- start QPME and create a new QPN file or open an existing file, which contains a subnet
- add a subnet to the newly created file
- open the editor for the added subnet
- call "File-->Save As"
- specify a (unique, not yet existing) name for the file
- click 'save'

Expected behavior:
- the file is stored to disk under the specified name or an error message is shown

Actual behavior:
- the 'save as' dialog closes without error, but the file is not created on disk

Suggested fix:
- implement functionality
- show error message