adobe_coeff: Nikon D5200 standard color matrix
fixed segfault when loading half-corrupted xmp
dng: don't delete meaningful exif in case of dng.
Don't think that images are hovered on empty space

this makes using the lighttable more "intuitive".
crop/rotate: fix mini-crop window on first selection.
blending: fix missing clamping in some blend modes that involve RGB<->HSL and Lab<->LCh
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)
wb_presets: Canon EOS REBEL T2i (correct case)
opencl: disable sm_20 for nvidia gts 240
Bump required version of glib to 2.28

We used the api from 2.28 already, so this shouldn't break anything,
just make it clear why a compile might fail later on.