widget: Rename focus_region (EO)
This region has little to do with focus, as it's more of a region of
interest within the widget, and not directly related to the highlight
geometry, for instance. It's related to focus in the sense that only
widgets with focus would really care about this region.
I decided to change this name after talking with @bu5hm4n.
Note that gengrid uses this but is also completely broken (the focus
highlight floats around and you don't even see the focused item).
Note: This is very close to show_region but I'm not sure those can be
merged safely (since the default "focus_region" is NULL while the
default "show_region" is the widget's geometry).