naughty: Add a title widget.

It is a normal textbox with some extra boilerplate code. Having this
in a separate widget allows the notification to be defined from rc.lua
without a ton of beautiful options and connect_signal.

lib/naughty/widget/init.lua

@@ -5,4 +5,5 @@
 ---------------------------------------------------------------------------
 
 return {
+    title = require( "naughty.widget.title"     );
 }

lib/naughty/widget/title.lua

@@ -0,0 +1,87 @@
+----------------------------------------------------------------------------
+--- A notification title.
+--
+-- This widget is a specialized `wibox.widget.textbox` with the following extra
+-- features:
+--
+-- * Honor the `beautiful` notification variables.
+-- * React to the `naughty.notification` object title changes.
+--
+--@DOC_wibox_nwidget_title_simple_EXAMPLE@
+--
+-- @author Emmanuel Lepage Vallee <elv1313@gmail.com>
+-- @copyright 2017 Emmanuel Lepage Vallee
+-- @widgetmod naughty.widget.title
+-- @see wibox.widget.textbox
+----------------------------------------------------------------------------
+local textbox = require("wibox.widget.textbox")
+local gtable  = require("gears.table")
+local beautiful = require("beautiful")
+
+local title = {}
+
+local function markup(notif, wdg)
+    local ret = "<b>"..(notif.title or "").."</b>"
+    local fg = notif.fg or beautiful.notification_fg
+
+    wdg:set_font(notif.font or beautiful.notification_font)
+
+    if fg then
+        ret = "<span color='" .. fg .. "'>" .. ret .. "</span>"
+    end
+
+    return ret
+end
+
+--- The attached notification.
+-- @property notification
+-- @tparam naughty.notification notification
+
+function title:set_notification(notif)
+    if self._private.notification == notif then return end
+
+    if self._private.notification then
+        self._private.notification:disconnect_signal("poperty::message",
+            self._private.title_changed_callback)
+        self._private.notification:disconnect_signal("poperty::fg",
+            self._private.title_changed_callback)
+    end
+
+    self:set_markup(markup(notif, self))
+
+    self._private.notification = notif
+    self._private.title_changed_callback()
+
+    notif:connect_signal("poperty::title", self._private.title_changed_callback)
+    notif:connect_signal("poperty::fg"   , self._private.title_changed_callback)
+end
+
+--- Create a new naughty.widget.title.
+-- @tparam table args
+-- @tparam naughty.notification args.notification The notification.
+-- @constructorfct naughty.widget.title
+
+local function new(args)
+    args = args or {}
+    local tb = textbox()
+    tb:set_wrap("word")
+    tb:set_font(beautiful.notification_font)
+
+    gtable.crush(tb, title, true)
+
+    function tb._private.title_changed_callback()
+        tb:set_markup(markup(tb._private.notification, tb))
+    end
+
+    if args.notification then
+        tb:set_notification(args.notification)
+    end
+
+    return tb
+end
+
+--@DOC_widget_COMMON@
+
+--@DOC_object_COMMON@
+
+return setmetatable(title, {__call = function(_, ...) return new(...) end})