meson: integrate wireplumber as a subproject

This allows building wireplumber as part of the pipewire build
and running it in the uninstalled environment instead of media-session.

Building each session manager is individually contolled by the options:
 -Dmedia-session=auto/enabled/disabled
 -Dwireplumber=auto/enabled/disabled

And controlling which one is used in pipewire.conf is done with:
 -Dsession-manager=media-session/wireplumber

Wireplumber's source tree must be in subprojects/wireplumber/
If this is missing, the .wrap file ensures that the latest git
master is downloaded while meson configures the build.
This git tree will not be automatically updated later, you need
to ensure that it is up-to-date on your own.

.gitignore

@@ -12,6 +12,9 @@ cscope.out
 cscope.in.out
 cscope.po.out
 Makefile
+subprojects/lua*
+subprojects/wireplumber
+subprojects/packagecache
 
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