msbuild: refactor to reduce rebuild events

* moves dolphin-specific settings out of Base.props
* creates exports.props for externals, allowing to easily import
  individual Externals
* corrects some cruft that accumulated and probably contributed
  to msbuild overbuilding

Externals/imgui/exports.props

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project>
+  <ItemDefinitionGroup>
+    <ClCompile>
+      <AdditionalIncludeDirectories>$(ExternalsDir)imgui;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
+    </ClCompile>
+  </ItemDefinitionGroup>
+  <ItemGroup>
+    <ProjectReference Include="$(ExternalsDir)imgui\imgui.vcxproj">
+      <Project>{4c3b2264-ea73-4a7b-9cfe-65b0fd635ebb}</Project>
+    </ProjectReference>
+  </ItemGroup>
+</Project>

Externals/imgui/imgui.vcxproj

@@ -15,6 +15,12 @@
     <Import Project="$(VSPropsDir)ClDisableAllWarnings.props" />
   </ImportGroup>
   <PropertyGroup Label="UserMacros" />
+  <ItemDefinitionGroup>
+    <ClCompile>
+      <!--IM_ASSERT is defined as Dolphin's ASSERT() :( (DolphinLib circular dependency)-->
+      <AdditionalIncludeDirectories>$(CoreDir);%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
+    </ClCompile>
+  </ItemDefinitionGroup>
   <ItemGroup>
     <ClCompile Include="imgui.cpp" />
     <ClCompile Include="imgui_draw.cpp" />
@@ -29,6 +35,7 @@
     <ClInclude Include="imstb_textedit.h" />
     <ClInclude Include="imstb_truetype.h" />
   </ItemGroup>
+  <Import Project="$(ExternalsDir)fmt\exports.props" />
   <Import Project="$(VCTargetsPath)\Microsoft.Cpp.targets" />
   <ImportGroup Label="ExtensionTargets">
   </ImportGroup>