GTK3: Do not use deprecated symbols. Part 1/2: GTK

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 GTK deprecated functions are disabled (-DGTK_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