Use segment terminology throughout segmented button APIs

SegmentedButton and ToolbarSegmentedButton APIs now describe their items as segments. Use AddSegment, SegmentCount, SegmentSymbolAt, SetSegmentSymbol and SelectedSegmentIndex, with equivalent segment names for all item operations. Existing row-named members remain available as deprecated aliases so existing projects continue to work. Studio-generated code, examples and documentation use the segment names.