awful.mouse: Add a "request::default_mousebindings" signal.

`rc.lua` and the module must attach to this signal to add buttons
to the default set.

lib/awful/mouse/init.lua

@@ -529,6 +529,15 @@ end)
 --  when button 1 is pressed.
 -- @staticfct mouse.coords
 
+capi.client.connect_signal("scanning", function()
+    capi.client.emit_signal("request::default_mousebindings", "startup")
+end)
+
+-- Private function to be used by `ruled.client`.
+function mouse._get_client_mousebindings()
+    return default_buttons
+end
+
 return mouse
 
 -- vim: filetype=lua:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:textwidth=80

objects/client.c

@@ -244,6 +244,17 @@
  * @signal request::urgent
  */
 
+/** Emitted during startup to gather the default client mousebindings.
+ *
+ * This signals gives a chance to all module to register new client keybindings.
+ * Assuming the client rules does not overwrite them with the `keys` property,
+ * they will be added to all clients.
+ *
+ * @signal request::default_mousebindings
+ * @tparam string context The reason why the signal was sent (currently always
+ *  `startup`).
+*/
+
 /** When a client gets tagged.
  * @signal tagged
  * @tag t The tag object.