geometry: added range_length().
tty: tidy.
geometry: split RangeRel::disjoint into disjoint_before and disjoint_after.
doc: added missing briefs and params; misc tidy.
txt::Cursor: commented-out and removed debug messages.
ui: corrected stdlib references to use aux instead.
test/general: added range test.
ui: added content_fg_selected and content_bg_selected properties.
ui: added ProtoGrid widget base class.
test/ui: added ProtoGrid test.