mu4e: implement mu4e-dbus-mode

For emacsen that support DBus, implement mu4e-dbus-mode, an experimental mode to
make some mu4e runtime information available outside emacs.

mu4e/meson.build

@@ -48,7 +48,6 @@ mu4e_srcs=[
   'mu4e-contrib.el',
   'mu4e-draft.el',
   'mu4e-folders.el',
-  'mu4e.el',
   'mu4e-headers.el',
   'mu4e-helpers.el',
   'mu4e-icalendar.el',
@@ -70,7 +69,8 @@ mu4e_srcs=[
   'mu4e-update.el',
   'mu4e-vars.el',
   'mu4e-view.el',
-  'mu4e-window.el'
+  'mu4e-window.el',
+  'mu4e.el'
 ]
 
 # emacs 28 is guaranteed to have transient
@@ -80,6 +80,13 @@ if emacs28.found()
   mu4e_srcs += 'mu4e-transient.el'
 endif
 
+# do have we have dbus support?
+emacs_dbus = run_command(emacs, '--batch', '--eval', '(kill-emacs (if (featurep \'dbusbind) 0 1))',
+                         check: false)
+if emacs_dbus.returncode() == 0
+  mu4e_srcs += 'mu4e-dbus.el'
+endif
+
 # note, we cannot compile mu4e-config.el without incurring
 #  WARNING: Source item
 #   '[...]/build/mu4e/mu4e-meta.el' cannot be converted to File object, because