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/glslang/glslang.vcxproj

@@ -15,6 +15,11 @@
     <Import Project="$(VSPropsDir)ClDisableAllWarnings.props" />
   </ImportGroup>
   <PropertyGroup Label="UserMacros" />
+  <ItemDefinitionGroup>
+    <ClCompile>
+      <AdditionalIncludeDirectories>.;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
+    </ClCompile>
+  </ItemDefinitionGroup>
   <ItemGroup>
     <ClCompile Include="glslang\GenericCodeGen\CodeGen.cpp" />
     <ClCompile Include="glslang\GenericCodeGen\Link.cpp" />