To get gnome-power-manager to properly recognize the backlight, to get desktop notifications for a change in brightness, and (atleast for me) to get maximum brightness, add the option acpi_backlight=vendor.
Make /etc/default/grub look like this:
Then sudo update-grub again.
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_osi=Linux acpi_backlight=vendor"
@Plippo: My wacom has been dead a week or two now. Years of use has frayed and loosened the cord, and I don't have the money for another, so it doesn't matter to me whether I can use a wacom with Mypaint. Still, there are others who want the option. Since they can't get support for both at the same time, maybe it could be an option to be turned off or on... I don't know. I don't pretend to know about such fancy things. But unless you're a Mypaint user too, I don't want you to worry about it. I can nag some other people, since it works so well