LibGfx: Improve ImageDecoder construction

Previously, ImageDecoder::create() would return a NonnullRefPtr and
could not "fail", although the returned decoder may be "invalid" which
you then had to check anyway.

The new interface looks like this:

    static RefPtr<Gfx::ImageDecoder> try_create(ReadonlyBytes);

This simplifies ImageDecoder since it no longer has to worry about its
validity. Client code gets slightly clearer as well.

Userland/Libraries/LibGUI/ImageWidget.cpp

@@ -78,7 +78,9 @@ void ImageWidget::load_from_file(const StringView& path)
         return;
 
     auto& mapped_file = *file_or_error.value();
-    m_image_decoder = Gfx::ImageDecoder::create((const u8*)mapped_file.data(), mapped_file.size());
+    m_image_decoder = Gfx::ImageDecoder::try_create(mapped_file.bytes());
+    VERIFY(m_image_decoder);
+
     auto bitmap = m_image_decoder->bitmap();
     VERIFY(bitmap);