New signal NeovimConnector::processExited

- When launching Neovim through ::spawn we need to know when the process
  exits, this signal is emitted when that happens
Close GUI when spawned Neovim exits
For Gcc/Clang be pedantic

- Brought back -Wall and -Wextra
Fix compiler warnings in Shell class
Return error for msgpack-rpc requests
Draw fullwidth chars

- Fullwidth chars take up two cells, Neovim signals this case
  by sending an empty char after a full width char
- The background painting for redraw:put notifications
  is now grouped into a single operation
- handlePut now only draws one char, that may leak into the
  next cell (because it is fullwidth)
Fix unused check expression

- For some reason GCC failed to warn about that statement, Clang
  caught it though.