Add gtk_cell_renderer_get_state()

This is a helper function to help retrieve a GtkStateFlags
from a GtkCellRendererState, also given the cell renderer
and widget sensitivities.

docs/reference/gtk/gtk3-sections.txt

@@ -4557,6 +4557,7 @@ gtk_cell_renderer_get_alignment
 gtk_cell_renderer_set_alignment
 gtk_cell_renderer_get_padding
 gtk_cell_renderer_set_padding
+gtk_cell_renderer_get_state
 gtk_cell_renderer_is_activatable
 
 <SUBSECTION Width-for-height>