Add implementation of missing methods of EmbeddedImageURLHandler

The code is restored from some older revision where it still
existed. Somehow it got lost over time, and since the handler
wasn't registered either the compiler completely ignored the
class so it did not complain.
2 jobs for master in 43 minutes and 15 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #75710
build_clazy_clang_tidy

00:25:23

passed #75709
linux kf5-qt5 qt5.12

00:17:52

35.0%