Split histogram into RGB colour channels

Instead of drawing a single histogram for the overall intensity,
draw separate graphs for each RGB colour channel. Also draw a
small 4x4 grid overlay.
Fix aperture and shutter speed metadata in status bar
Only import known image extension in the session

Instead of importing all files to a session, only import files
whose extension is found in a whitelist. This ensures we only
get image files loaded
Ensure check for raw files is case insensitive

When checking the filename extension to identify raw files, convert
the filename to lowercase first, to ensure the check is case
insensitive
Set sane default values for processing raw files

Set some default parameters for libraw so that it processes files
using the typical camera defaults. This should give better results,
particularly with Canon raw files.
Try to extract preview/thumbnail from raw files

The exiv2 data stream doesn't always contain good thumbnails for
raw files. Try to extract a preview/thumbnail from the raw file
first, then try exiv2, and finally fallback to resizing the main
image
Set prefs before loading session

Starting up the session was loaded before the preferences were
applied. Thus, the first image loaded would be processed using
the wrong settings.
Apply orientation from EXIF data to images

If GTK fails to apply any orientation data from the image file,
then fallback to applying orientation data from the exif
metadata block
Refresh translations from transifex
Remove leftover g_printerr statements