Tabs, dividers, file tree and sidebar, polished

Fixed

• Tab close buttons. The X closes a tab on the first click, including a half-hidden tab next to the scroll arrow, and the whole corner around it counts.
• Dragging tabs. A dragged tab stays under the pointer while the strip scrolls, the strip scrolls at an even speed at its ends, and chat tabs scroll too. Escape cancels a drag, and switching windows mid-drag no longer drops the tab somewhere.
• Scrolling tabs. A trackpad swipe no longer keeps drifting after you stop, a mouse wheel moves the strip straight away, and the scroll arrows appear as soon as a tab grows past the edge. The open-tabs list tells apart files with the same name, and middle-click closes Changes, Files and the other panel tabs.
• Panel and split dividers. Escape or switching windows puts a divider back where it started, a release outside the window ends the drag, and a right-click no longer starts one. The Code Intel divider drags the right way and stops at its limit. Double-click a split divider to even out its panes.
• File tree drag and drop. Moving an item onto itself, into its own folder or into one of its subfolders is refused, the drop highlight no longer sticks, folders open when you hover over them, and the tree scrolls near its edges. Files can be moved back to the top level, and a file dropped on the editor opens there.
• File tree keyboard and rename. Home, End and Space work, keys in the right-click menu stay in the menu, Enter and Escape in a rename do what they say, and renaming readme.md to README.md works. Opening a folder no longer jumps the tree back to the active file.
• Sidebar. Escape cancels a workspace drag, and the “…” and Stop buttons on agent rows no longer miss clicks. Titles use the full row width, rename handles Escape and IME input properly, search shows every match, and the arrow keys move through the whole list.