Use Glib::ustring::collate_key() for ThumbBrowserEntryBase::operator<()

Kudos to @heckflosse for having a nose for optimizations. 👍

Also kills unused `DirEntry` class.

rtgui/thumbbrowserentrybase.h

@@ -88,6 +88,9 @@ protected:
     // called during updateBackBuffer for custom overlays
     virtual void customBackBufferUpdate (Cairo::RefPtr<Cairo::Context> c) {}
 
+private:
+    const std::string collate_name;
+
 public:
 
     Thumbnail* thumbnail;
@@ -169,7 +172,7 @@ public:
 
     bool operator <(const ThumbBrowserEntryBase& other) const
     {
-        return shortname.casefold() < other.shortname.casefold();
+        return collate_name < other.collate_name;
     }
 
     ThumbBrowserEntryBase* getOriginal () const;