[FIX] mail: save scroll position of chat window when folded

- Remove no more used ThreadViewer prop : messageListInitialScrollTop
- Remove no more used ChatWindow prop : threadInitialScrollTop
- Rename ThreadViewer field threadCacheInitialPosition to
  threadCacheInitialScrollPosition
- Use the scroll position to set message list scroll on mounted
- Fix old test which was not simulating correctly home menu show
- Add tests to check that scroll position is correctly saved even if
  ChatWindow is folded
- Fix traceback when showing/hiding home menu with a folded chat window

task-2285691

closes odoo/odoo#54198

X-original-commit: 31123a94c156424b3962fe4aebc3cc60a7a89223
Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>