More uses of AUDACITY_DLL_API...

... in many places where the function call will later need to be between
modules (or libraries, or the executable) and the annotation will be a necessity
to keep the linkage working on Windows.

That's all that this sweeping commit does.

include/audacity/PluginInterface.h

@@ -51,7 +51,7 @@
 class ModuleInterface;
 
 
-class PluginManagerInterface /* not final */
+class AUDACITY_DLL_API PluginManagerInterface /* not final */
 {
 public:
 

include/audacity/Types.h

@@ -287,7 +287,7 @@ using CommandIDs = std::vector<CommandID>;
 // The msgid should be used only in unusual cases and the translation more often
 //
 // Implicit conversions to and from wxString are intentionally disabled
-class TranslatableString {
+class AUDACITY_DLL_API TranslatableString {
    enum class Request;
    template< size_t N > struct PluralTemp;