Consider using system menu for "typical" functionality

[daemontus]

Right now, we have import/export tab which is ok, but not very idiomatic in desktop apps. Instead, we could use system menus for this, as other applications do. As part of this, we could also "improve" the recent/last model functionality to be more in line with typical behavior in desktop apps.

[daemontus]

What we know so far:

• On macOS, help should ideally be last, otherwise stuff breaks.
• On macOS, the first item should be "app menu" with about/quit/etc.
• We should test menu show/hide on macOS to see if it would be possible to have different per-window menus.
• If show/hide does not work, we need to make a menu on macOS which changes items using enabled/disabled depending on the focused window.

[daemontus]

Mostly resolved by #15, but there are still some issues/inconsistencies to look out for, so I'll keep this open for brainstorming.

TODOs:

• It would make sense to enable the import/export menu also in other windows, in which case the export would be the underlying model (for computation) and the import would simply open a new editor.
• It would be nice to have help with manual as the last menu, because then we get a "search menus" functionality in macOS for free.
• We should add .bnet import, but I'll take care of that.