Avalonia: Make MONITORINFO a struct

[wieslawsoltes]

This is possible fix for exception thrown when using CoreRT.

Feedback from CoreRT chatroom :

"Btw, you can work around the GetMonitorInfo trouble by making MONITORINFO a struct and passing it either as a pointer or a ref to the GetMonitorInfo method. The compiler can't generate marshaling for classes currently"

https://github.com/AvaloniaUI/Avalonia/blob/a0331b98896bffa09f02b0db999527fdf0fe2481/src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs#L1058

Error when using WindowStartupLocation:

Unhandled Exception: System.Exception: Method '[Avalonia.Win32]Avalonia.Win32.Interop.UnmanagedMethods.GetMonitorInfo(native int,MONITORINFO)' requires non-trivial marshalling that is not yet supported by this compiler.
   at Avalonia.Win32.Interop.UnmanagedMethods.GetMonitorInfo(IntPtr, UnmanagedMethods.MONITORINFO) + 0x23
   at Avalonia.Win32.ScreenImpl.<>c__DisplayClass4_0.<get_AllScreens>b__0(IntPtr, IntPtr, Rect&, IntPtr) + 0x82
   at Core2D.Avalonia!<BaseAddress>+0x1475a4b

Error when using menus:
```
Unhandled Exception: System.Exception: Method '[Avalonia.Win32]Avalonia.Win32.Interop.UnmanagedMethods.GetMonitorInfo(native int,MONITORINFO)' requires non-trivial marshalling that is not yet supported by this compiler.
at Avalonia.Win32.Interop.UnmanagedMethods.GetMonitorInfo(IntPtr, UnmanagedMethods.MONITORINFO) + 0x23
at Avalonia.Win32.ScreenImpl.<>c__DisplayClass4_0.b__0(IntPtr, IntPtr, Rect&, IntPtr) + 0x82
at Core2D.Avalonia!+0x1475a4b
````

[jmacato]

@wieslawsoltes we might as well do a Avalonia.Native port for Windows so that we dont have to deal with marshalling to and from native code :)

[wieslawsoltes]

@jmacato In long term that would be the best and complete solution, for today we can fix small issues I think :smile:

[jmacato]

@wieslawsoltes i definitely agree :smile: so for now we can change MONITORINFO into a struct... can you pop a PR for that bud?

[jkoritzinsky]

re porting the Avalonia.Win32 API to native: I'd rather keep it in managed code if possible, at least for the windowing. When we add an accessibility layer we'll probably need to add that to Avalonia.Native though.

[danwalmsley]

Avalonia native could probably just supply the monitor info stuff and like you say accessibility. Just where managed code cant be used for whatever reason.

[ahopper]

This error also occurs when you mouse over a tooltip

[kekekeks]

Avalonia.Native is meant for platforms where it's too hard to properly interop with the native code. Cocoa and Qt are good examples. In case of Win32/GTK/X11 and even Wayland it would be easier to maintain managed code base, since they use plain C API.

[kekekeks]

Regarding MONITORINFO structure: it was probably made a class to initialize cbSize from constructor. I don't see any other reason why it should be a class.