T22: Collaboration

- presence: avatars ringed in each peer's color with where they are, press to
  follow; a breathing live dot; follow mode frames the view and stops on Escape
- remote cursors and selections painted over a field from its own layout
- comment threads with replies, reactions and resolve; a marker that opens a
  thread in a bubble; a sidebar of open and resolved threads
- annotations washed in their authors' colors, notes on hover
- suggestion mode: edits become suggestions word by word; track changes shows
  them struck and underlined, to accept or reject
- sharing: invite by email with checked chips, roles, the access list and who
  the link lets in, in one dialog
- primitives::hold_focus gives a mode its own focus, shared with ZenMode
- AvatarGroup's tuck and count shared; Avatar::ring_color; TagInput::check;
  TextInput::bounds_for_range; Popover::size
- icons: smile-plus, user-plus, message-square-diff
- gallery: the Collaboration page with scripted states

examples/gallery/pages/mod.rs

@@ -1,5 +1,6 @@
 mod buttons;
 mod charts;
+mod collab;
 mod data;
 mod debug;
 mod documents;
@@ -57,6 +58,7 @@ pub const ALL: &[Page] = &[
     git::PAGE,
     debug::PAGE,
     documents::PAGE,
+    collab::PAGE,
     theme::PAGE,
 ];