Add i3 native support + deprecate --wrapper

[meator]

As far as I know the only use of --wrapper is to do i3 exec. Therefore this generic feature has only one use and it's failing at it (this is further described in #132).

I propose adding direct support for i3 through i3 IPC. This feature would be fully optional, I myself don't use i3 so j4dd won't depend on it.

I will look up if any other window manager has similar mechanism which would warrant having --wrapper. If not, it will be deprecated in release 3.0 (it will work but it will emit a warning to stderr). It would be removed in a future version then.

Even if the --wrapper option would stay, adding i3 support would still be beneficial. It would mean tighter support for i3, faster execution of programs and more. j4-dmenu-desktop is based on i3-dmenu-desktop so having i3 support would "pay homage to its ancestors". If --wrapper won't be deprecated, a warning shall be issued if j4dd is executed with --wrapper i3\ exec. The warning would direct the user to the new i3 integration.

[meator]

--wrapper will not be deprecated in r3.0 release.