Made lensfun lookups for one lens really unique.  Fixes bug #9662.
Add keywords, as recommended here:

 http://standards.freedesktop.org/desktop-entry-spec/latest/ar01s05.html,
 http://bugs.debian.org/693918, and
 https://wiki.gnome.org/Initiatives/GnomeGoals/DesktopFileKeywords
Proper english. 'allows one to', not 'allows to'
Keywords value MUST end with a semicolon
initial implementation of slideshow view
slideshow: coarse view skeleton
slideshow: create background thread to process image buffer.
slideshow: fix crash.
slideshow: comments about the plan
slideshow: get screen dimensions and alloc double buffer
slideshow: fix mem allocation
slideshow: process images, click left and right
slideshow: use display profile
slideshow: add some locks around thread shared data.

also center image.
slideshow: remove debug output
slideshow: hide borders around image
slideshow: fix centering issue
slideshow: stupid synchronization for backbuf processing.

also fix alignment problem of dt_log

also broke right mouse click (go back) again, since next image is now
already processed.
slideshow: todo about async picture advancing
slideshow: introduce state machine to sync bg job and ui