Sweep through UI strings to internationalize

[PIG208]

Fixes:

• Sweep through UI strings to internationalize #277

[gnprice]

Thanks!

This is a lot fewer new strings than I'd expected it to be. I guess we've been pretty good about i18n in new code for the past year or so.

I did part of a sweep through git grep '"' lib and git grep "'" lib and found a few more (amongst lots of results in comments and in non-user-facing or otherwise invariant strings):

lib/widgets/content.dart
1576:      TextSpan(text: "(unimplemented:", style: errorStyle),
1578:      TextSpan(text: ")", style: errorStyle),
1582:      TextSpan(text: "(unimplemented: text «", style: errorStyle),
1584:      TextSpan(text: "»)", style: errorStyle),
1588:      text: "(unimplemented: DOM node type ${htmlNode.nodeType})",

lib/widgets/lightbox.dart
391:        semanticsLabel: "Current position"),
421:        semanticsLabel: "Video duration"),

lib/widgets/message_list.dart
429:          return Text("DMs with ${names.join(", ")}"); // TODO show avatars
648:            child: Text("No earlier messages."))); // TODO use an icon
699:        tooltip: "Scroll to bottom",
1147:        .join(", "));

lib/licenses.dart
26:      return '$licenseFileText\n\nAUTHORS file follows:\n\n$authorsFileText';

lib/model/compose.dart
148:  return '${mention(sender!, silent: true)} ${inlineLink('said', url)}: ' // TODO(i18n) ?
149:    '*(loading message ${message.id})*\n'; // TODO(i18n) ?
172:    return '${mention(sender!, silent: true)} ${inlineLink('said', url)}:\n' // TODO(i18n) ?

(Stopping there to switch to another task — the remaining code for that sweep is in the "'" grep, starting from lib/model/content.dart.)