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/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>