Prepare to split AudioIOBase from AudioIO...

... New files, but (almost) empty; don't use the global variable gAudioIO,
but use one of two accessor function names (which are the same function for
now).

AudioIOBase will have fewer dependencies than AudioIO -- in particular, no
dependency on tracks.

It won't include StartStream.  It will contain functions to query the
present state of streams, and device capabilities.

win/Projects/Audacity/Audacity.vcxproj

@@ -129,6 +129,7 @@
     </ClCompile>
     <ClCompile Include="..\..\..\src\AudacityLogger.cpp" />
     <ClCompile Include="..\..\..\src\AudioIO.cpp" />
+    <ClCompile Include="..\..\..\src\AudioIOBase.cpp" />
     <ClCompile Include="..\..\..\src\AutoRecovery.cpp" />
     <ClCompile Include="..\..\..\src\AutoRecoveryDialog.cpp" />
     <ClCompile Include="..\..\..\src\BatchCommandDialog.cpp" />
@@ -506,6 +507,7 @@
     <ClInclude Include="..\..\..\src\AudacityHeaders.h" />
     <ClInclude Include="..\..\..\src\AudacityLogger.h" />
     <ClInclude Include="..\..\..\src\AudioIO.h" />
+    <ClInclude Include="..\..\..\src\AudioIOBase.h" />
     <ClInclude Include="..\..\..\src\AudioIOListener.h" />
     <ClInclude Include="..\..\..\src\AutoRecovery.h" />
     <ClInclude Include="..\..\..\src\AutoRecoveryDialog.h" />

win/Projects/Audacity/Audacity.vcxproj.filters

@@ -128,6 +128,9 @@
     <ClCompile Include="..\..\..\src\AudioIO.cpp">
       <Filter>src</Filter>
     </ClCompile>
+    <ClCompile Include="..\..\..\src\AudioIOBase.cpp">
+      <Filter>src</Filter>
+    </ClCompile>
     <ClCompile Include="..\..\..\src\AutoRecovery.cpp">
       <Filter>src</Filter>
     </ClCompile>
@@ -1213,6 +1216,9 @@
     <ClInclude Include="..\..\..\src\AudioIO.h">
       <Filter>src</Filter>
     </ClInclude>
+    <ClInclude Include="..\..\..\src\AudioIOBase.h">
+      <Filter>src</Filter>
+    </ClInclude>
     <ClInclude Include="..\..\..\src\AutoRecovery.h">
       <Filter>src</Filter>
     </ClInclude>