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

Apt-offline checks if the `install` and `set` option can be used on the current system, when using the CLI version.
However, it only checks those two operations for Unix on the GUI.

So instead of getting the following error when using the GUI version on Windows:

````
    if os.geteuid() != 0:
AttributeError: 'module' object has no attribute 'geteuid'
````

It now shows the error message:
`This is supported only on Unix like systems with apt installed.`