Tab groups with a tab strip drawn like the system tab bar

[Zedyas]

Summary

Adds Chrome-style tab groups. The system window tab bar cannot show a clickable group chip, hide tabs, or take clicks on its labels, so each window now draws its own tab strip, measured to match the system bar.

• Tabs: each window holds its tabs (ReaderTab, one per document) and shows one. Menu and toolbar commands pass on to the shown tab through the responder chain.
• Strip: equal-width tabs on a band that darkens the window color. The shown tab stays clear so it joins the page. Titles are centered, the close button appears at the left on hover, and a new tab button sits at the right end. It follows light, dark, and inactive windows.
• Groups: a group has an optional name and a color from the app's own palette. Its chip is a tinted segment before its tabs, and a line in its color runs under them.
  • Click the chip to collapse or expand the group; a collapsed chip shows a tab count.
  • Right-click the chip to rename, recolor, add a tab, ungroup, close, or move the group to a new window.
  • Right-click a tab to add it to a group or take it out.
• Dragging: a tab or chip follows the pointer along the strip while the other tabs slide aside. Pulled away from the strip, it drags to another window's strip, or opens a new window when released outside every window.
• Sessions: groups, including collapsed ones, are saved and restored with the session. Sessions saved before this change still load.
• Shortcuts: ⌘W closes a tab and ⇧⌘W the window. ⇧⌘[ ⇧⌘], ⌃⇥ ⌃⇧⇥, and ⌘1–⌘9 skip tabs hidden in collapsed groups. View > Always Show Tab Bar (⇧⌘T).

Testing

• 21 Swift tests pass. They cover strip order and drop slots, keeping groups together, session filtering and decoding of older sessions, and moves and closes that keep the shown tab and collapsed groups.
• Checked by mouse in a test copy with its own saved data:
  • Hover and close.
  • The new tab button, including with tabs overflowing a narrow window.
  • Creating, naming, collapsing, and recoloring a group.
  • Dragging a tab into a group's end and its middle, and dragging a group by its chip.
  • Tearing a tab off into a new window, and dropping a tab onto another window's strip.
  • ⌘W while dragging, and 15 tabs in one window.
  • Quitting and relaunching with groups.
• The strip's colors were compared pixel by pixel with the system tab bar in light and dark.