[WIP] Remove FOR_ALL_WINDOWS and replace with FOR_ALL_WINDOWS_IN_TAB(curtab)

`FOR_ALL_WINDOWS` suggests that all windows are iterated over. In reality it is just the windows in the current tab. Replacing makes the behavior clearer.

Also removed `W_NEXT(wp)` since it was used in two places and doesn't buy anything over the `wp->w_next` that it expands to.
@splinterofchaos That change is actually part of https://github.com/neovim/neovim/pull/1108 that seems like probably a good idea though.