Implement backend for notification settings and buffer mutes

[justjanne]

In Short

• allows setting a buffer separately to notify on all/highlighted/no messages, or to use the default for that buffer type
• allows muting a buffer until a certain timestamp
• allows setting a default for queries, channels and status buffers

TODO

• Add a utility function to the class to determine if a message should cause a notification or not, to handle this consistently between core and client (for future core-side notifications)
• Make the Qt client filter notifications based on these settings
• Add UI to the Qt client to configure notification settings and mute duration

Impact

Criteria	Rank	Reason
Impact	★☆☆ 1/3	Improves situations where notifications for specific buffers could not be turned off separately
Risk	★☆☆ 1/3	Does not touch any critical code
Intrusiveness	★☆☆ 1/3	Changes localized to a few classes

Rationale

Being able to set certain levels of notification settings is a relatively important feature, as users might like to be notified on all messages in a channel without having them highlighted, or might like to mute a chat for a certain time, or disable notifications entirely for a certain bot (e.g. the quite annoying Q bot on Quakenet).

This adds a functionality which is common already in most other chat clients, and is one of the most popular feature requests and support topics in Quasseldroid