gpuicn v0.7.0

gpuicn v0.7.0

This release closes the gaps Matty found in the chat components, and adds a
composer, an emoji picker and a lightbox. Runtime pins are unchanged from
v0.6.0: GPUI Kit 0.8.1 on gpui-pre 0.3.10, and gpui-icons 0.5.0.

New components

• Composer: a growing message box. Enter sends and Shift+Enter adds a
  line. It can read and set the selection, insert at the caret, wrap a
  selection, report the caret's rectangle, and tell when an input method is
  composing. Key and paste hooks run before the editor, so the app can claim
  pasted images and files. @, ~, : and / open an autocomplete popup
  above the trigger, filled with suggestions from the app.
• Emoji Picker: search from two characters, category tabs, a virtualized
  grid with keyboard navigation, skin tones, and frequently used, recently
  used and custom emoji supplied by the app. It reports the picked shortcode.
• Lightbox: a full-window image viewer with zoom, pan, previous and next
  from the keyboard and mouse, caller toolbar actions and focus return.

Changes to existing components

• Message: reaction chips can show a custom emoji image and a who-reacted
  tooltip (reaction_tooltip takes any view). There is a new
  MessageHighlight::Permalink and a per-row aria_description.
• Message List: an opt-in keyboard selection mode (track_focus,
  selected, on_select, selectable, on_key).
• Markdown: selection_root mounts window text selection, so a drag
  selects across messages and Cmd+C or Ctrl+C copies it. CustomEmoji draws
  inline emoji images at text height, and MarkdownImages sends every image
  through the app's resolver.
• Command: set_leading and set_trailing slots, for avatars and unread
  badges.

Every image slot takes an ImageSource from the app; gpuicn never fetches.

Upgrading

Message now depends on Tooltip in the registry. Copied-source apps should run
gpuicn diff, then gpuicn add message --overwrite (which brings in
tooltip), before adding the new components. Without selection_root,
Markdown in a gpuicn app can't be selected.