[RFC] FOR_ALL_TAB_WINDOWS cleanup. Add FOR_ALL_TABS and FOR_ALL_WINDOWS_IN_TAB.

Adds `FOR_ALL_TABS` and `FOR_ALL_WINDOWS_IN_TAB`. These combine together to make the `FOR_ALL_TAB_WINDOWS` helper that already existed.
```c
      bool is_mark_set = true;
      if (buf->b_last_cursor.lnum == 0) {
        is_mark_set = false;
```