Implemented `:only` for splits `C-W C-O` and `C-W C-W`.

Added the following command and key mappings:

- `:wincmd[!] o` to remove all splits by either moving all editors into
  the current split or (with `!`) closing other editors. Mapped to `C-W o`
  and `C-W C-O`.
- `:wincmd w` to switch to most recently active split. The behaviour is
  different to VIM: in VIM `C-W C-W` cycles through all the splits
  moving down/right. To implement that would require editor coordinates
  which is not trivial to obtain.