Add Qt submodule for windows.

.gitmodules

@@ -0,0 +1,4 @@
+[submodule "Externals/Qt"]
+	path = Externals/Qt
+	url = https://github.com/dolphin-emu/ext-win-qt.git
+	branch = master

Source/VSProps/QtCompile.props

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project ToolsVersion="12.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <PropertyGroup Label="UserMacros">
-    <QTDIRDefault>$(ExternalsDir)Qt5.3.1\5.3\msvc2013_64_opengl\</QTDIRDefault>
+    <QTDIRDefault>$(ExternalsDir)Qt\Qt5.3.1\5.3\msvc2013_64_opengl\</QTDIRDefault>
     <QTDIR Condition="Exists('$(QTDIRDefault)') And '$(QTDIR)'==''">$(QTDIRDefault)</QTDIR>
     <QTDIR Condition="Exists('$(QTDIR)') And !HasTrailingSlash('$(QTDIR)')">$(QTDIR)\</QTDIR>
     <QtDirValid>false</QtDirValid>