Open .atnx project files on all platforms

[netherwaves]

Describe the bug
Currently, project files can only be opened from within the application. ATNX file extensions are not associated to autonomX by default, and when forcibly opened in autonomX from the Explorer, they aren't actually being opened, instead showing the factory state of the application. This can quickly be annoying from a UX perspective, because users are forced into a single method of save state management, which isn't necessarily the most convenient in a majority of cases.

[aalex]

We need to create 3 tickets for this: one for each platform.

[netherwaves]

@aalex thanks! please refer to #390, #391 and #392 for platform-dependent task tickets

[aalex]

Let's convert this ticket to an epic!