Different design: more compact windows and collapsing/closing control on each tab bar

[micfong-z]

Resolves #238. See the issue for a full discussion of this design.

Here's a list of technically missing features in this PR, but I'd like to hear some opinions before carrying on ;)

• [x] ~~Proper styling (especially rounding)~~
  • After careful checking of the design, this should be done on the user side.
• [x] Dragging tabs to an empty space on the main surface (https://github.com/Adanos020/egui_dock/pull/237/commits/fd805c924e2febed9bebee02c338d3ee0bf840cd, https://github.com/Adanos020/egui_dock/pull/237/commits/ccce02f21005aada100e33c47a6b00c5b4d55e1b)
• [x] Dragging tabs to a collapsed Leaf (https://github.com/Adanos020/egui_dock/pull/237/commits/fd805c924e2febed9bebee02c338d3ee0bf840cd, https://github.com/Adanos020/egui_dock/pull/237/commits/ccce02f21005aada100e33c47a6b00c5b4d55e1b)
• [x] Proper CHANGELOG (https://github.com/Adanos020/egui_dock/pull/237/commits/370e477ff33af9565bed560b3c23f9cfdf6aa7a5)
• [x] Update examples (https://github.com/Adanos020/egui_dock/pull/237/commits/6e56846f1203627b17fb2d12c480c380676ec13f)