Fix comparison of empty revision with last one

revision can be None and so can not be compared to a datetime
Clear entry when combobox is set to inactive item

GTK doesn't clear the entry when calling set_active with -1.

issue4298
review13691002
get_focus_chain can return None

issue4318
review5721002
Use the parent children for prefetching only works for same model

otherwise it must use the direct group

issue4319
review5721003
Separate hue color of scheme by golden angle

issue4257
review12631002