rc.lua: Do not hardcode the default client buttons.

This commit moves the previously hardcoded client button list
to the `awful.client` default button set. This will allow modules
to add their own buttons.

In later commit, those default buttons will also be added dynamically
to existing clients.

awesomerc.lua

@@ -437,7 +437,8 @@ awful.keyboard.append_global_keybindings({
 })
 
 -- @DOC_CLIENT_BUTTONS@
-clientbuttons = {
+client.connect_signal("request::default_mousebindings", function()
+    awful.mouse.append_client_mousebindings({
         awful.button({ }, 1, function (c)
             c:emit_signal("request::activate", "mouse_click", {raise = true})
         end),
@@ -449,7 +450,8 @@ clientbuttons = {
             c:emit_signal("request::activate", "mouse_click", {raise = true})
             awful.mouse.client.resize(c)
         end),
-}
+    })
+end)
 
 -- }}}
 
@@ -465,7 +467,6 @@ awful.rules.rules = {
                      focus = awful.client.focus.filter,
                      raise = true,
                      keys = clientkeys,
-                     buttons = clientbuttons,
                      screen = awful.screen.preferred,
                      placement = awful.placement.no_overlap+awful.placement.no_offscreen
      }