Remove redundant toplevel save state logic
Add is_screen_tall_enough function and error mess
Remove s:minwidth variable
Fix sidebar_size() to use the text area width
Fix CenterScreen to obey width and height setting

- Width and sidebar height settings weren't being implemented correctly because
  splitting and then moving the window caused all windows (sidebars and vimroom
  window) to be of equal size instead of the size they were supposed to be set
  to.
Make sidebar buffers unreachable

- Make sidebar buffers unlisted so that they can't be accessed from buffer list
- Set sidebar buffers to be wiped out when hidden
- Set autocmd so that trying to switch to sidebar buffers leaves the cursor in
  the current window