JXR: added support to EXIF metadata

Improved metadata support via EXIF ​​metadata. Since JXR is based on a TIFF container, EXIF ​​data is read directly from the file so it always works (even with versions of libjxr that don't have the metadata reading API).

It also solves the following issues:
- Incorrect date format on saved JXR files (was saved in ISO format instead of `yyyy:MM:dd HH:mm:ss`).
- Incorrect date type setting in EXIF ​​data: the `DateTime` tag should be updated on every save (verified by GIMP and Photoshop). Our `CreationDate` metadata is the equivalent of the EXIF ​​`DateTimeOriginal` tag.

Closes #22

src/imageformats/CMakeLists.txt

@@ -140,7 +140,7 @@ endif()
 ##################################
 
 if (LibJXR_FOUND)
-    kimageformats_add_plugin(kimg_jxr SOURCES jxr.cpp)
+    kimageformats_add_plugin(kimg_jxr SOURCES jxr.cpp microexif.cpp)
     kde_enable_exceptions()
     target_include_directories(kimg_jxr PRIVATE ${LIBJXR_INCLUDE_DIRS})
     target_link_libraries(kimg_jxr PRIVATE ${LIBJXR_LIBRARIES})