Preliminary documentation of provider interfaces

* Preliminary documentation of CallsCall and CallsMessageSource
* Documentation and cleaning up of CallsCallData
* Add data files and meson rules to create gtk-doc documentation
* Move some macros from util.h to calls-message-source.h

doc/meson.build

@@ -0,0 +1,31 @@
+if get_option('gtk_doc')
+
+subdir('xml')
+
+glib_prefix = dependency('glib-2.0').get_pkgconfig_variable('prefix')
+glib_docpath = join_paths(glib_prefix, 'share', 'gtk-doc', 'html')
+docpath = join_paths(get_option('datadir'), 'gtk-doc', 'html')
+
+gnome.gtkdoc('calls',
+           main_xml: 'calls-docs.xml',
+            src_dir: [
+                       join_paths(meson.source_root(), 'src'),
+                       join_paths(meson.build_root(), 'src'),
+                       join_paths(meson.build_root(), 'libgdbofono'),
+                     ],
+       dependencies: calls_deps,
+          scan_args: [
+                       '--rebuild-types',
+                     ],
+       fixxref_args: [
+                       '--html-dir=@0@'.format(docpath),
+                       '--extra-dir=@0@'.format(join_paths(glib_docpath, 'glib')),
+                       '--extra-dir=@0@'.format(join_paths(glib_docpath, 'gobject')),
+                       '--extra-dir=@0@'.format(join_paths(glib_docpath, 'gio')),
+                       '--extra-dir=@0@'.format(join_paths(glib_docpath, 'gi')),
+                       '--extra-dir=@0@'.format(join_paths(glib_docpath, 'gtk3')),
+                     ],
+        install_dir: 'calls',
+            install: true)
+
+endif