Execute user provided command invocations $CMD such as `editor_cmd` with
`/bin/sh` as `/bin/sh -c "$CMD"
Previously, user commands were split by whitespace which must trigger
erroneous behavior if quotes are involved.
Merge ui crate with root crate.
In preparation for uploading `meli` as a separate crate on crates.io.
Workspace crates will need to be published as well and having a separate
`ui` crate and binary perhaps doesn't make sense anymore.
