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/cpp-optparse/exports.props

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project>
+  <ItemDefinitionGroup>
+    <ClCompile>
+      <AdditionalIncludeDirectories>$(ExternalsDir)cpp-optparse;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
+    </ClCompile>
+  </ItemDefinitionGroup>
+  <ItemGroup>
+    <ProjectReference Include="$(ExternalsDir)cpp-optparse\cpp-optparse.vcxproj">
+      <Project>{c636d9d1-82fe-42b5-9987-63b7d4836341}</Project>
+    </ProjectReference>
+  </ItemGroup>
+</Project>