conf_override! wraps struct definitions and defines a secondary Override
struct that wraps each field in an Option. The macro mailbox_settings!
is used to select settings from an account & mailbox index. If a user defines an overriding setting, the macro returns the override instead of the immediately next in the hierarchy setting.
The selection is done for a specific field as follows:
if per-folder override is defined, return per-folder override
else if per-account override is defined, return per-account override
else return global setting field value.
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.
Manos Pitsidianakis