Revamp packaging to achieve more correct Lua style
Several changes regarding packaging, namespacing and handler API:
* Utility function module is now called 'imapdomo', not 'common'.
* The module behaves as one, instead of being just a global-modifying
include.
* Lots of cleanups as advised by luacheck (local declarations,
removal of unused variables...)
* Handlers now have to access utility API via `require` and provide
accout configuration in new file mailboxes.lua, which has to return
table. Accessing mailboxes is via utility function. (This sounds
complicated but actually is not. Believe me! :))
Having just 4 valid actions (albeit they do form a meaningful workflow)
was rather historical limitation caused by author's negligence.
User should be able to define any actions they want by simply creating
handlers for them.
Later we might want to provide a guidance (template) for that, but
that's a TODO item.