prevent avahi signal racing

Summary:
https://github.com/lathiat/avahi/issues/9

Avahi has upstream signal races which can have any number of side effect
bugs on our end ranging from unreliable discovery, over missing servers
in kio slave listings, to outright deadlocking when waiting for a result
which has already raced passed us (specifically RemoteService is vulnerable
to that).

When we make a request to avahi it will immediately start sending
discovery signals even when we haven't even connected to the signals yet.
This means any number of signals may be lost in the time between our
request and us connecting to the signals.
As this applies to (all?) in-the-wild versions of avahi even if we got an
upstream fix today it'd not help the user until they get the new avahi
integrated.

To prevent us from losing the race all dbus signal listening is now done
via blanket connects.
Ordinarily we'd make a request, dbus would give us an object path, we'd
connect to the object path's signals and then get signals for our request.
Since the signals are racey we'll instead connect to all signals of a given
interface (e.g. org.freedesktop.Avahi.DomainBrowser) completely ignoring
the object path. We'll then "locally" filter all signals which belong to
the specific DomainBrowser object as identified by the signals' dbus object
path. This eliminates all risk of racing. Before we make the request
we setup our blanket connections and the slots won't get called until after
the event loop returns, which won't be until we've set "our" dbus object
path for filtering later in the relevant functions.

The obvious downsides of this are:

• boilerplatey code for handling this stuff
• runtime checked signal-slot compatibility
• much more traffic going into our slots (doesn't actually end up doing any heavy lifting)

Test Plan: - wrote a crappy test app to use all browser -> still browse (also service resolves)

Reviewers: mdawson, broulik, Frameworks

Reviewed By: broulik

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D16298