Add menu item options into config

[crspybits]

Client apps can now either (a) supply their own menu icons, or (b) entirely use their own icons and actions (through use of the ..navigationBarItems modifier).

I refactored the code some too. Sorry it was a fairly large refactor as Xcode/SwiftUI was causing me difficulties and it made debugging the problem easier to split things out into more independent View's. I think it does, however, improve readability.

This should (hopefully!) be a non-breaking change. Other applications using this package should still work with no change.

[crspybits]

This should resolve: https://github.com/Vidhyadharan24/SideMenu/issues/5

[Vidhyadharan24]

@crspybits that's a lot of work, I will have a look at it.

[crspybits]

Happy to answer any questions you might have about this PR. I'm actively using my changed fork of your repo in https://apps.apple.com/us/app/neebla/id1244482164 (and see https://github.com/SyncServerII/Neebla/). Would be happy to switch back to this repo if this was merged in.