Move source files into src directory (oops), and fix struct field warnings.

* Warnings fixed by adding [StructLayout(LayoutKind.Sequential)].

src/ImGui.NET/ImGui.NET.csproj

@@ -16,10 +16,6 @@
     <DocumentationFile Condition="'$(Configuration)' == 'Release'">$(OutputPath)\ImGui.NET.xml</DocumentationFile>
     <RootNamespace>ImGuiNET</RootNamespace>
   </PropertyGroup>
-  <ItemGroup>
-    <Compile Include="..\..\NativeDrawContext.cs" Link="NativeDrawContext.cs" />
-    <Compile Include="..\..\NativeSimpleColumns.cs" Link="NativeSimpleColumns.cs" />
-  </ItemGroup>
   <ItemGroup>
     <PackageReference Include="System.Numerics.Vectors" Version="4.4.0" />
     <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="4.4.0" />

src/ImGui.NET/NativeContext.cs

@@ -1,8 +1,10 @@
 using System;
 using System.Numerics;
+using System.Runtime.InteropServices;
 
 namespace ImGuiNET
 {
+    [StructLayout(LayoutKind.Sequential)]
     public unsafe struct NativeContext
     {
         public byte Initialized;

src/ImGui.NET/NativePayload.cs

@@ -1,5 +1,8 @@
-namespace ImGuiNET
+using System.Runtime.InteropServices;
+
+namespace ImGuiNET
 {
+    [StructLayout(LayoutKind.Sequential)]
     public unsafe struct NativePayload
     {
         public void* Data;

src/ImGui.NET/NativeWindow.cs

@@ -1,7 +1,9 @@
 using System.Numerics;
+using System.Runtime.InteropServices;
 
 namespace ImGuiNET
 {
+    [StructLayout(LayoutKind.Sequential)]
     public unsafe struct NativeWindow
     {
         public byte* Name;