TreeBrowser: Fix GTK+3 support

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
GeanyLua: Fix GTK+3 support

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
PrettyPrinter: Fix GTK+3 support

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>