Bug Description

Follow-up to #19 / #282: The current Dominant Color module enforces usage of its extended WP_Image_Editor classes. While this works with WordPress core alone, it is problematic in environments where e.g. a plugin has already added its own extended WP_Image_Editor classes.

We need to make sure that the Dominant Color implementation doesn't conflict with such plugins. The dominant_color_set_image_editors() filter callback function should be fixed to not override the entire array of editor class names, but rather only replace the specific WordPress core editors WP_Image_Editor_GD and WP_Image_Editor_Imagick with their customized variants from the module.

cc @pbearne @spacedmonkey