[iOS] Manual .sln and .csproj migration for VS2017 (Windows) compatibility

[FreezyLemon]

VS2017 wanted to migrate the .sln files (because of "Visual Studio 2012" in the header) and the .csproj files (because of the iOS.props Import not being at the top and somehow not working properly).

Automatic migration broke a lot of stuff though, so I did it manually. VS2017 still changes a lot of things when loading, but most are just "cosmetics" like rearranging build configs.

One of the devs on Mac should test this to see if it opens and builds properly.

• Dependent on [iOS] Manual .sln and .csproj migration for VS2017 (Windows) compatibility osu-framework#1429

[Frontear]

I'm curious as to why this would benefit. Xamarin building for iOS requires a MAC, and if you own a mac, you may as well be cloning and building on the mac as is. Windows users just can't do anything useful with this.

[FreezyLemon]

@Frontear Multiple things:
Xamarin.iOS builds do not require a Mac, though testing them does require one (or an iPhone, not sure). This also has virtually no negative effects on Mac users if done right, as I will be leaving minor (non breaking) changes that only VS on Windows makes uncommited.

As to why I would want to fix this, there are two main reasons:

1. It is currently planned to merge most (or all) branches into master, which would include iOS. I'd like to think that it would be nice to have the .iOS.sln files working on windows aswell when this happens.
2. Mobile development on Android is also going to happen, and it's very possible that there's some shared code between the two mobile platforms, in which case it would be nice to access the iOS stuff on windows.

[FreezyLemon]

This gives a bit more context on what's going on here, by the way. It seems the project GUID that is currently used is a legacy option, and the one I replaced it with is for newer .NET Core projects (like the ones we are using so the project finds the files to compile without explicit declaration).

[smoogipoo]

Prior to this, we need to update the iOS branch with the latest master changes since netstandard has brought a few sln changes.

Feel free to do so and apply these sln changes both on master and on the iOS branches (and then re-PR). Closing this one for now.

(Preferrably apply on master first).