Add client.first_tag, as a shortcut for `c:tags()[1]`

This is meant to be a faster alternative in case only the first tag is
relevant/used.

Closes https://github.com/awesomeWM/awesome/pull/294.

awesomerc.lua

@@ -141,7 +141,7 @@ mytasklist.buttons = awful.util.table.join(
                                                   -- :isvisible() makes no sense
                                                   c.minimized = false
                                                   if not c:isvisible() then
-                                                      awful.tag.viewonly(c:tags()[1])
+                                                      awful.tag.viewonly(c.first_tag)
                                                   end
                                                   -- This will also un-minimize
                                                   -- the client, if needed

lib/awful/client.lua

@@ -61,7 +61,7 @@ function client.jumpto(c, merge)
     end
 
     -- Try to make client visible, this also covers e.g. sticky
-    local t = c:tags()[1]
+    local t = c.first_tag
     if t and not c:isvisible() then
         if merge then
             t.selected = true

lib/awful/menu.lua

@@ -613,7 +613,7 @@ end
 --       terms[i] =
 --         {c.name,
 --          function()
---            awful.tag.viewonly(c:tags()[1])
+--            awful.tag.viewonly(c.first_tag)
 --            client.focus = c
 --          end,
 --          c.icon

objects/client.c

@@ -97,6 +97,7 @@
  * @field shape_client_clip The client's clip shape as set by the program as a (native) cairo surface.
  * @field startup_id The FreeDesktop StartId.
  * @field valid If the client that this object refers to is still managed by awesome.
+ * @field first_tag The first tag of the client.  Optimized form of `c:tags()[1]`.
  * @table client
  */
 
@@ -1559,9 +1560,11 @@ luaA_client_swap(lua_State *L)
 
 /** Access or set the client tags.
  *
- * @param tags_table A table with tags to set, or none to get the current tags
+ * Use the `first_tag` field to access the first tag of a client directly.
- *  table.
+ *
- * @return A table with all tags.
+ * @tparam table tags_table A table with tags to set, or `nil` to get the
+ *   current tags.
+ * @treturn table A table with all tags.
  * @function tags
  */
 static int
@@ -1611,6 +1614,23 @@ luaA_client_tags(lua_State *L)
     return 1;
 }
 
+/** Get the first tag of a client.
+ */
+static int
+luaA_client_get_first_tag(lua_State *L)
+{
+    client_t *c = luaA_checkudata(L, 1, &client_class);
+
+    foreach(tag, globalconf.tags)
+        if(is_client_tagged(c, *tag))
+        {
+            luaA_object_push(L, *tag);
+            return 1;
+        }
+
+    return 0;
+}
+
 /** Raise a client on top of others which are on the same layer.
  *
  * @function raise
@@ -2639,6 +2659,10 @@ client_class_setup(lua_State *L)
                             NULL,
                             (lua_class_propfunc_t) luaA_client_get_client_shape_clip,
                             NULL);
+    luaA_class_add_property(&client_class, "first_tag",
+                            NULL,
+                            (lua_class_propfunc_t) luaA_client_get_first_tag,
+                            NULL);
 
     /** When a client gains focus.
      * @signal .focus