Add a notification rate limiter.

## Prerequisites
- [x] I have searched open and closed issues for duplicates.

## Feature
**Is your feature request related to a problem? Please describe.**
Technically, yes. The problem is that a malicious application can crash the entire system by spamming notifications. Just had this with a Wine program happen.

**Describe the solution you'd like**
A rate limiter can do the job. The server will refuse to handle any new notifications from a process for some time, if that process already sent over for example 20 notifications within a minute. Every minute that counter resets and the process can start sending notifications again.

The only problem is how would a developer would handle such an error. Afaik `libnotify` at least would have to be modified, I don't know if DBus would too.

**Existing work**
Maybe, but I don't know.

**Describe alternatives you've considered**
None apart from just letting someone's OS to crash.

**Additional context**
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