Import the WebP image I/O code from kde-runtime

The plugin export mechanism has been patched up (including the addition
of the JSON file), and the FindWebP.cmake file is new.

Writing is currently disabled, as it produces broken images.

Autotests are generated using the cwebp and dwebp utilities distributed
with the libwebp reference library.

REVIEW: 115355

autotests/CMakeLists.txt

@@ -53,6 +53,7 @@ kimageformats_read_tests(
     rgb
     tga
     xcf
+    webp
 )
 
 # Basic write tests