ipc: implement GET_VERSION to find out the i3 version

ipc: implement GET_VERSION to find out the i3 version

This is useful for third-party scripts which require certain features
and want to error out cleanly when they are run with an old i3 version.

Additionally, i3 --version might be different from what’s actually
running (an old version of the binary), so i3-msg -t get_version will be
the best way to figure out the i3 version you are actually running from
this commit on.