[VM] Any interest in trying to make local vs. IMAP folder treatment more seamless

[blueman]

For example, it would be nice to have a single vm-visit-any-folder
function which would visit local vs. IMAP folders depending on whether
there is a ':' (colon) in the name.

For example:
    'foldername' = local folder 'foldername
    ':foldername' = IMAP folder on default imap server
    'server:foldername' = IMAP folder on IMAP server 'server'
    'folder\:name' = local folder 'foldername'

Similarly, having a single 'FCC' line that would be treated as IMAP or
local folders depending on the name of the header contents

Similarly, for other key frequently used functions.

This collapsing would be especially useful given the limited number of
key bindings so that for example 'v' can be used to visit any (IMAP or
local) folder.