If `OnlyFetchMvbox` is enabled, `MvboxMove` ("Move automatically to DeltaChat folder") makes no
sense: useful messages from "Spam" are moved anyway to "DeltaChat" (see
`imap::spam_target_folder_cfg()`) and Inbox messages aren't fetched and moved. Then `MvboxMove` can
mean "Move Inbox and all scanned folders' messages to DeltaChat", so if it's enabled, Inbox is
watched too, so the user can configure fetching only from "Inbox", "DeltaChat" and "Spam" which was
unachievable before and helps e.g. for the Gmail case which has many virtual folders and the whole
folder scanning process is really slow (and useless for most users):
https://github.com/chatmail/core/issues/7178#issuecomment-3266343253. This may also be useful if the
user has another messenger, e.g. a Delta Chat fork who moves messages to "ZetaChat": this folder
shouldn't be scanned, but Inbox should be. Mvbox is watched anyway, this should just be made clear
to users (if not already).
For existing users accidentally having both options enabled "MvboxMove" is disabled in a migration
to not cause watching Inbox.
Having both options disabled means that we don't separate chat messages from non-chat ones, scanning
all folders is fine in this case.
1cf067c045