Support new event types

[nilmerg]

The type column of the event table now has a defined type which only allows certain values:

• acknowledgement-cleared
• acknowledgement-set
• custom
• downtime-end
• downtime-removed
• downtime-start
• flapping-end
• flapping-start
• incident-age (previously internal)
• state

Previously only the following values where used:

• state
• internal (now incident-age)

We need to support the new values, remove all workarounds (internal support, message matching) and show proper phrases:

Type	Phrase
acknowledgement-cleared	was unacknowledged
acknowledgement-set	has been acknowledged
custom	no phrase needed
downtime-end	left a downtime period
downtime-removed	prematurely left a downtime period
downtime-start	entered a downtime period
flapping-end	left a flapping period
flapping-start	entered a flapping period
incident-age	exceeded a time constraint
state	depending on severity, unchanged

[yhabteab]

There are also two new event types (mute, unmute) since Icinga/icinga-notifications#200.

[nilmerg]

This is a very different change. A separate issue will follow.

[yhabteab]

They are not that different, at least in regard to the event table. These two events are also part of the event.type enum.