BlockData::to_markdown and from_markdown: a document of blocks as GitHub Markdown and back, each field's inline Markdown kept as written (#7)

tasks/github.md

@@ -20,6 +20,6 @@
 - [x] G12 Issue #7: an IME asks for its candidate box past the laid-out text; the box clamps to the last layout, as `bounds_for` does; tested
 - [x] G13 Issue #12: a floating option list occludes, so the wheel scrolls the list and not the page under it; tested
 - [x] G14 Issue #7: BlockEditor scrolls in its own `ScrollArea` with its scrollbar and keeps the caret in view as blocks are added; tested
-- [ ] G15 Issue #7: BlockEditor turns into Markdown and back; tested
+- [x] G15 Issue #7: BlockEditor turns into Markdown and back; tested. Five reviews, the cap: the fifth finding (a slash ended an unquoted HTML value) is fixed and tested, unreviewed
 - [ ] G16 Issue #7: a selection across blocks: drag or Shift-press selects, copy, cut, delete and typing act on it, undo restores; tested
 - [ ] G17 Push, reply on #7 and #12