Don't hard-code the exhaustive list of sub-view types...

... in Wave track context menu and SetTrackVisualsCommand

Instead, discover them through a registry.

This eliminates some duplication of string constants and prepares for
non-intrusive generalization to more kinds of sub-views.

This makes the command agnostic about which subview types are known, but the
context menu still has special case treatment for Spectrogram Settings and
Wave Colors.

include/audacity/ComponentInterface.h

@@ -87,9 +87,10 @@ public:
 
    const wxString &Internal() const { return mInternal; }
    const TranslatableString &Msgid() const { return mMsgid; }
+   const TranslatableString Stripped() const { return mMsgid.Stripped(); }
    const wxString Translation() const { return mMsgid.Translation(); }
    const wxString StrippedTranslation() const
-      { return mMsgid.Stripped().Translation(); }
+      { return Stripped().Translation(); }
 
    bool empty() const { return mInternal.empty(); }