Import.cpp does not depend on subclasses of ImportPlugin...

... freeing 15 files from the big strongly connected component:

ImportFFmpeg & ODDecodeFFMpegTask (still in a cycle of two)
ImportFLAC
ImportGStreamer
ImportLOF
ImportMIDI
ImportMP3
ImportOGG
ImportPCM
ImportQT
ProjectFileManager
ProjectFSCK
ProjectManager
ProjectSelectionManager
ODDecodeFlacTask

And eight header files in src/import are deleted.

This breaks a lot of cycles because Import, which is still in the big component,
no longer includes ImportLOF, which recursively uses ProjectManager. A
registration system for the subclasses of ImportFileHandle allows that
recursion without the cyclic compilation dependencies.

win/Projects/Audacity/Audacity.vcxproj

@@ -543,7 +543,6 @@
     <ClInclude Include="..\..\..\src\HitTestResult.h" />
     <ClInclude Include="..\..\..\src\import\FormatClassifier.h" />
     <ClInclude Include="..\..\..\src\import\ImportForwards.h" />
-    <ClInclude Include="..\..\..\src\import\ImportGStreamer.h" />
     <ClInclude Include="..\..\..\src\import\MultiFormatReader.h" />
     <ClInclude Include="..\..\..\src\import\SpecPowerMeter.h" />
     <ClInclude Include="..\..\..\src\InconsistencyException.h" />
@@ -752,13 +751,7 @@
     <ClInclude Include="..\..\..\src\export\ExportOGG.h" />
     <ClInclude Include="..\..\..\src\export\ExportPCM.h" />
     <ClInclude Include="..\..\..\src\import\Import.h" />
-    <ClInclude Include="..\..\..\src\import\ImportFFmpeg.h" />
-    <ClInclude Include="..\..\..\src\import\ImportFLAC.h" />
-    <ClInclude Include="..\..\..\src\import\ImportLOF.h" />
     <ClInclude Include="..\..\..\src\import\ImportMIDI.h" />
-    <ClInclude Include="..\..\..\src\import\ImportMP3.h" />
-    <ClInclude Include="..\..\..\src\import\ImportOGG.h" />
-    <ClInclude Include="..\..\..\src\import\ImportPCM.h" />
     <ClInclude Include="..\..\..\src\import\ImportPlugin.h" />
     <ClInclude Include="..\..\..\src\import\ImportRaw.h" />
     <ClInclude Include="..\..\..\src\import\RawAudioGuess.h" />