image: Add newly designed thumbnail for DnD

The old method stopped working with the new decoder setup.

The new design features a frame for contrast and rotates
images based on EXIF information.

Closes #89

src/widgets/image_page.rs

@@ -173,10 +173,6 @@ impl LpImagePage {
         self.imp().image.get()
     }
 
-    pub fn texture(&self) -> Option<gdk::Texture> {
-        self.imp().image.texture()
-    }
-
     pub fn content_provider(&self) -> Option<gdk::ContentProvider> {
         self.imp().image.content_provider()
     }