DisplayConfig: Make naming convention consistent

Always use hyphens as delimit words in property keys. Underscore to
delimit words in method arguments.

https://bugzilla.gnome.org/show_bug.cgi?id=777732

src/backends/meta-monitor-manager.c

@@ -1456,13 +1456,13 @@ meta_monitor_manager_handle_get_current_state (MetaDBusDisplayConfig *skeleton,
           gboolean is_underscanning = meta_monitor_is_underscanning (monitor);
 
           g_variant_builder_add (&monitor_properties_builder, "{sv}",
-                                 "is_underscanning",
+                                 "is-underscanning",
                                  g_variant_new_boolean (is_underscanning));
         }
 
       is_builtin = meta_monitor_is_laptop_panel (monitor);
       g_variant_builder_add (&monitor_properties_builder, "{sv}",
-                             "is_builtin",
+                             "is-builtin",
                              g_variant_new_boolean (is_builtin));
 
       g_variant_builder_add (&monitors_builder, MONITOR_FORMAT,
@@ -1544,7 +1544,7 @@ meta_monitor_manager_handle_get_current_state (MetaDBusDisplayConfig *skeleton,
                              max_screen_height);
 
       g_variant_builder_add (&properties_builder, "{sv}",
-                             "max_screen_size",
+                             "max-screen-size",
                              g_variant_builder_end (&max_screen_size_builder));
     }
 

src/org.gnome.Mutter.DisplayConfig.xml

@@ -323,15 +323,15 @@
 	    * d preferred scale: scale preferred as per calculations
 	    * u flags: mode flags (see below)
 	* a{sv} properties: optional properties, including:
-	    - "width_mm" (i): physical width of monitor in millimeters
-	    - "height_mm" (i): physical height of monitor in millimeters
-	    - "is_underscanning" (b): whether underscanning is enabled
+	    - "width-mm" (i): physical width of monitor in millimeters
+	    - "height-mm" (i): physical height of monitor in millimeters
+	    - "is-underscanning" (b): whether underscanning is enabled
 				      (absence of this means underscanning
 				      not being supported)
-	    - "max_screen_size" (ii): the maximum size a screen may have
+	    - "max-screen-size" (ii): the maximum size a screen may have
 				      (absence of this means unlimited screen
 				      size)
-	    - "is_builtin" (b): whether the monitor is built in, e.g. a
+	    - "is-builtin" (b): whether the monitor is built in, e.g. a
 				laptop panel (absence of this means it is
 				not built in)