GTK3: Use cairo for drawing

In GTK+ 3, the GDK drawing API (which closely mimics the X drawing API, which is itself modeled after PostScript) has been removed. All drawing in GTK+ 3 is done via cairo.

The GdkGC and GdkImage objects, as well as all the functions using them, are gone. This includes the gdk_draw family of functions like gdk_draw_rectangle() and gdk_draw_drawable(). As GdkGC is roughly equivalent to cairo_t and GdkImage was used for drawing images to GdkWindows, which cairo supports automatically, a transition is usually straightforward.

See https://developer.gnome.org/gtk3/3.9/gtk-migrating-2-to-3.html#id-1.6.3.3.8
Renamed darktable.gui->s to darktable.gui->surface
Merge pull request #311 from MRIG/master

update dutch translations
brush and path masks: fix corner deletion
rawspeed: update to svn r588 (thanks klaus)
rawspeed: add back aspect ratio support for the Pana DMC-G6 and minor tweak for DMC-GH3
wb_presets: Canon PowerShot SX50 HS (stolen from UFRaw)
CR distortion management : no need to round values
brush masks: increased reserve in buffer management
usermanual: some fixes for drawn masks
Allow to force the way to get the display profile
rawspeed: Canon EOS 6D height fix
Fix compile when not using colord
wb_presets: Adding whitebalance-presets for Pentax K-30 (extracted from DNG)
Update French translation.
Update French translation.

Shorten a translation that otherwise gets cut on some systems.
brush masks: fix potential crash
brush masks: sync payload and border in case of sharp edges
brush masks: minor fix
brush masks: fix another small rendering issue (gap)