Add missing gdk required dependency for champlain, other related fixups

meson: Add missing gdk required dependency for champlain

gdk/gdk.h header is included and gdk_cairo_set_source_pixbuf symbol is used from gdk, thus we need its header include path and link to it. If clutter is built with gtk support, this is implicitly included via clutter-1.0 dep, but it can also be built without gtk support, in which case the build of libchamplain fails due to no gdk/gdk.h header found.

Clean up gdk.h vs gdk-pixbuf.h includes

Some source files included gdk.h without any use; others included it instead of gdk-pixbuf/gdk-pixbuf.h