find volume feedback's runtime requirements

Summary:
I'm in a good mood so there's even finders!

- FindCanberra now also sets the version (this is necessary to hint
  directories for the pulse backend lookup)
- new FindCanberraPulse attempts to find the canberra-pulse backend using
  manually constructed hinting (unfortunately canberra's pc doesn't
  define the dir so we'll basically have to guess)
- new FindSoundThemeFreedesktop which locates the freedesktop sound theme
  we actually require for the notification sound (since we have no plasma
  sound theme :()

The new finders are put to use to locate and indicate RUNTIME dependencies.
As with all runtime deps, if the finders should proof unreliable
CMAKE_DISABLE_FIND_PACKAGE_* can be used to easily silence the warnings.
They may of course also be safely ignored.

Getting canberra and the sound-theme dependency right has proven a
challenge to distributions. The problem goes back even to when kde-runtime
was still a thing and used canberra, and AFAIK wasn't even resolved by
all distributions by the time plasma 5 came around and kde-runtime killed
off. So, clearly we need to help things along a bit by properly codifying
and communicating the requirements here.

The volume feedback is effectively disabled when the pulse backend cannot
be loaded, and the actual feedback sound is by default coming from the
default xdg theme which also must be available for the feedback to work.

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18807