The GUI now outputs an error message on Windows for Unix-only operations

Apt-offline checks the `install` and `set` option, when using the CLI version.
However, this doesn't happen on the GUI.

So instead of getting the following error:

````
    if os.geteuid() != 0:
AttributeError: 'module' object has no attribute 'geteuid'
````

The GUI now outputs:
`This is supported only on Unix like systems with apt installed.`