sheets: one Sheet component, and the info sheet over a document

Every overlay is drawn in Sheet.svelte: the header, the close button, the
footer of keys and the list cursor kept on screen, written once instead of
in ten copies. Overlays open and close through openSheet and closeSheet.

The outline, history and backlinks sheets and the share bar are one info
sheet with a part for each, Tab between them. gi opens it, gO, gh and gb open
it at their part, and so does a click on the line under the title. The
buttons in the document header are gone; the meta line says when the
document is public, and a phone gets edit and info under the text.

docs/wiki/Spaces-and-Documents.md

@@ -66,12 +66,17 @@ linked to from anywhere behind the login.
 ![A document in the reader](../images/document.png)
 
 The line under the title says when it was written and last edited, how many
-words it has, and how many documents link to it. In the reader:
+words it has, how many documents link to it, and whether it is public. Click it,
+or press `gi`, for the info sheet: the document's headings, its earlier
+versions, the documents that link to it and its share link, one part at a time
+— `Tab` goes to the next. `gO`, `gh` and `gb` open it at their own part. In the
+reader:
 
 | key | what it does |
 | --- | --- |
 | `j` / `k`, `Ctrl-D` / `Ctrl-U` | scroll a line, or half a page |
 | `]]` / `[[` | the next / previous heading |
+| `gi` | the info sheet |
 | `gO` | the headings as a list to jump to |
 | `zl` / `zh` | scroll a wide table or code block sideways |
 | `F` | a letter over every link and button in view; type it to follow |