thumbnailer: save the original image size into the thumbnail
Simplified the image preloader

Removed the two steps policy that loaded the image at the original
size after loading it at the requested size.  Now it only loads the
image at the requested size without the second step.
image viewer: adapt to the preloader changes

store the requested size in the image history to keep track when
a better quality is needed by the viewer
slideshow: adapt to the new image preloader
browser: load the metadata after a timeout
removed debug code
preloader: reload the image if the cached image isn't exactly the same size
do not load a better quality a new image has been requested
preloader: set an error if the operation wasn't cancelled
browser: do not load the metadata if the image changed
removed unnecessary braces
desktop background: fixed creation of a new filename
image viewer page: added async function to get the original image
adjust colors: use the async task to get the original image
preloader: remove the source if set
fixed error when getting the modified image

return a GthImage instead of a cairo_surface_t
added a GthImageViewerTask to simplify the code

GthImageViewerTask is a GthImageTask that loads the original
image from a GthImageViewerPage object.
adjust colors: use GthImageViewerTask to load the image
adjust contrast: use GthImageViewerTask to load the image
adjust colors: removed unused variable