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/minizip/exports.props

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project>
+  <ItemDefinitionGroup>
+    <ClCompile>
+      <AdditionalIncludeDirectories>$(ExternalsDir)minizip;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
+    </ClCompile>
+  </ItemDefinitionGroup>
+  <ItemGroup>
+    <ProjectReference Include="$(ExternalsDir)minizip\minizip.vcxproj">
+      <Project>{23114507-079a-4418-9707-cfa81a03ca99}</Project>
+    </ProjectReference>
+  </ItemGroup>
+</Project>

Externals/minizip/minizip.vcxproj

@@ -57,6 +57,7 @@
     <None Include="LICENSE" />
     <None Include="README" />
   </ItemGroup>
+  <Import Project="$(ExternalsDir)zlib-ng\exports.props" />
   <Import Project="$(VCTargetsPath)\Microsoft.Cpp.targets" />
   <ImportGroup Label="ExtensionTargets">
   </ImportGroup>