Make textbox widget respect the current theme

Without this fix `wibox.widget.textbox` ignores current theme font
setting and resets it to the initial one.
It becomes handy when the initial theme is tweaked during runtime, ie.
in `rc.lua`. Other AwesomeWM components like `awful` (taglist,
tasklist, menu) are aware and do respect the actual theme settings.

Closes https://github.com/awesomeWM/awesome/pull/89.

lib/wibox/widget/textbox.lua

@@ -229,7 +229,7 @@ local function new(text, ignore_markup)
     ret:set_wrap("word_char")
     ret:set_valign("center")
     ret:set_align("left")
-    ret:set_font()
+    ret:set_font(beautiful and beautiful.font)
 
     if text then
         if ignore_markup then