Make textclock garbage-collectable

This fixes the textclock-specific part of the test that the previous commit
added.

To fix this, gears.timer.weak_start_new() is used. This function creates a timer
that is automatically stopped when its callback function is garbage collected.
The callback function is saved as a member of the texbox widget that is the
"widget behind the textclock". Thus, the timer can only be stopped after the
widget is garbage-collected, but the timer does not keep the widget alive.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/awful/widget/textclock.lua

@@ -31,13 +31,14 @@ function textclock.new(format, timeout)
     local timeout = timeout or 60
 
     local w = textbox()
-    local t = timer { timeout = timeout }
-    t:connect_signal("timeout", function()
+    local t
+    function w._textclock_update_cb()
         w:set_markup(DateTime.new_now_local():format(format))
         t.timeout = calc_timeout(timeout)
         t:again()
-    end)
-    t:start()
+        return true -- Continue the timer
+    end
+    t = timer.weak_start_new(timeout, w._textclock_update_cb)
     t:emit_signal("timeout")
     return w
 end