#81161 by inactivist, Scale watermark to be proportional to image width

#142178 by schnizZzla, Upgrade to image 5x 1.1 breaks watermark png path
#153995 by schnizZzla, various fixes:
  new:
  - Preserve image type
  - Alpha blending
  - Apply watermark on uploaded images only
  - Watermark on previews
  - Toggles: Exclude gallery terms
  changes:
  - visible name is now "Image Watermark", project name is still "watermark"
  - image creation process has been slightly changed to transform images to truecolor first
  - Watermark settings moved to "Image Watermark" menu. This is a change due to the
    behaviour of image module that uses _image_build_derivatives every time settings has been saved.
    This caused watermarks to disappear or to appear on all image sizes.