opencl: disable sm_20 for nvidia gts 240
fix small memleak when importing
make colorpicker of tonecurve disappear immediately in the image after deactivation

put dt_control_queue_redraw() at a more reasonable position

(squashed into one to not leave the infinite loop as a separate commit)
Fixed up/down arrow keys in fullscreen preview mode and made left/right arrow keys work the same way. Also implemented the "center scroll" function, which realigns the images with the grid so there is no empty space in front of the first image.