GTK3: Do not use deprecated symbols. Part 2/2: GDK

Over the years, a number of functions, and in some cases, entire widgets have been deprecated.
These deprecations are clearly spelled out in the API reference, with hints about the recommended replacements.
The API reference for GTK+ 2 also includes an index of all deprecated symbols.

In this part, only GDK deprecated functions are disabled (-DGDK_DISABLE_DEPRECATED -Werror=implicit-function-declaration)

See https://developer.gnome.org/gtk3/3.9/gtk-migrating-2-to-3.html#id-1.6.3.3.4
usermanual: more on "collect images"
Merge pull request #302 from LebedevRI/gtk3-02-do-not-use-deprecated-symbols-part-2-of-2

GTK3: Do not use deprecated symbols. Part 2/2: GDK