* Not everything compiles; all protocols, and inbound filters do, though.
* Renamed a few classes to give a better idea what they are for; prefixed
public classes with the 'B' prefix.
* Moved ProtocolConfigView's classes into the BPrivate namespace.
* Moved BMailFilter into its own file.
* Added BMailFilter::DescriptiveName(). This is now used by the RuleFilter
in order to give a description of what it's doing (ie. no more dozens of
"Rule filter" entries in the preferences).
* Removed no longer used MailAddon.h.
* Renamed Addon to AddOn where found, since that is more consistent with the
rest of the API.
* Merged the former MailProtocol with the former MailProtocolThread; the
differentiation between those two was pretty messy.
* All configuration views touched so far are now using the layout kit.
* The RuleFilter is currently broken functionality wise; I have not yet decided
how to solve the stuff it uses (TriggerFileMove() does not exist anymore,
for example).
* BMailAddOnSettings (formerly known as AddonSettings) now directly subclass
BMessage; there are no Settings() and EditSettings() method anymore. The
class uses a copy of itself to determine whether or not it has been changed.
* Lots of cleanup.
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