Add msbuild target for building translations. Closes #1076.

aegisub/build/Aegisub/Aegisub.targets

@@ -185,4 +185,15 @@
     <ProjectTools Include="AegisubConfig" />
   </ItemGroup>
 
+  <!-- Compile translations if msgfmt is available -->
+  <Target Name="Translations"
+    AfterTargets="ClCompile"
+    Condition="'$(MsgFmtPath)'!=''"
+    Inputs="@(MsgFmt)"
+    Outputs="$(AegisubBinaryDir)locale\%(Filename)\aegisub.mo"
+    >
+    <Error Condition="!Exists($(MsgFmtPath))" Text="$(MsgFmtPath) not found" />
+    <MakeDir Directories="$(AegisubBinaryDir)locale\%(MsgFmt.Filename)" />
+    <Exec Command="$(MsgFmtPath) -o $(AegisubBinaryDir)locale\%(MsgFmt.Filename)\aegisub.mo %(MsgFmt.Identity)" />
+  </Target>
 </Project>

aegisub/build/Aegisub/Aegisub.vcxproj

@@ -459,7 +459,10 @@
   <ItemGroup>
     <ResourceCompile Include="$(SrcDir)res.rc" />
   </ItemGroup>
+  <ItemGroup>
+    <MsgFmt Include="$(AegisubSourceBase)po\*.po" />
+  </ItemGroup>
   <ImportGroup Label="ExtensionTargets">
     <Import Project="$(MSBuildThisFileDirectory)Aegisub.targets" />
   </ImportGroup>
-</Project>
+</Project>

aegisub/build/Aegisub/Aegisub.xml

@@ -78,6 +78,12 @@
      DisplayName="Update Checker URL"
      Description="URL to get for updates."
      />
+   <StringProperty
+     Name="MsgFmtPath"
+     Category="Config"
+     DisplayName="msgfmt.exe location"
+     Description="Location of msgfmt.exe, for compiling translations"
+     />
 
    <!-- External Libraries -->
   <BoolProperty