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

src/primitives/icon.rs

@@ -187,6 +187,7 @@ icons! {
     Menu => "menu",
     Merge => "merge",
     MessageSquare => "message-square",
+    MessageSquareDiff => "message-square-diff",
     MicOff => "mic-off",
     Mic => "mic",
     Minimize2 => "minimize-2",
@@ -242,6 +243,7 @@ icons! {
     SlidersHorizontal => "sliders-horizontal",
     Smartphone => "smartphone",
     Smile => "smile",
+    SmilePlus => "smile-plus",
     Sparkles => "sparkles",
     Spline => "spline",
     SquareCheck => "square-check",
@@ -267,6 +269,7 @@ icons! {
     Undo2 => "undo-2",
     Upload => "upload",
     User => "user",
+    UserPlus => "user-plus",
     Users => "users",
     Variable => "variable",
     VideoOff => "video-off",