Add pulseaudio gsettings schema

g_settings_new asserts the gsettings schema is installed and compiled, and this is not the case on any system where pulseaudio has been replaced with pipewire-pulse, so pactl load-module module-gsettings causes the server to core dump.

I've added the schema from upstream (https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/blob/master/src/modules/gsettings/org.freedesktop.pulseaudio.gschema.xml) and updated meson.build to output it. As an aside I noticed here that the if statement is "gio_dep.found" - I think this would be true even if get_option('gsettings') is false. So I think this would result in anyone manually compiling this not having the option of disabling gsettings if GIO is available on their system (even though setting gsettings = false might give them the impression of having that option.)

I wasn't sure if there is some way to ensure changes to the upstream gschema make their way into this repo - or whether that's desirable.

I used the meson gnome module's post_install function because it takes care of not running the script if --destdir is used when installing, i.e. if the app is being packaged. I checked and this is compatible with the currently specified minimum meson version.

Notes for packagers

• usr/share/glib-2.0/schemas/org.freedesktop.pulseaudio.gschema.xml should be picked into the pulse subpackage.
• If you allow pipewire-pulse and pulseaudio to be installed at the same time, both packages will attempt to install usr/share/glib-2.0/schemas/org.freedesktop.pulseaudio.gschema.xml. You could use -D gsettings-pulse-schema=disabled to stop pipewire-pulse installing it and then create (from pulseaudio) some common dependency package like gsettings-pulseaudio-schema to provide this file to both packages.

Related to: #1066