naughty: Add a shared default widget_template for the notifications.

This will be shared by the notifications stored in a wibox/wibar and the
ones using a popup. It extends the constraint and margins container
to take care of some boilerplate code. While other widgets have their
own public API, those 2 are private since they are not different enough
to warrent a new public module.

lib/naughty/widget/_default.lua

@@ -0,0 +1,85 @@
+----------------------------------------------------------------------------
+--- The default widget template for the notifications.
+--
+-- @author Emmanuel Lepage Vallee <elv1313@gmail.com>
+-- @copyright 2019 Emmanuel Lepage Vallee
+-- @classmod naughty.widget._default
+----------------------------------------------------------------------------
+
+local wibox      = require("wibox")
+local actionlist = require("naughty.list.actions")
+local wtitle     = require("naughty.widget.title")
+local wmessage   = require("naughty.widget.message")
+local wicon      = require("naughty.widget.icon")
+local wbg        = require("naughty.container.background")
+local beautiful  = require("beautiful")
+local dpi        = require("beautiful").xresources.apply_dpi
+
+-- It is not worth doing a special widget for this.
+local function notif_size()
+    local constraint = wibox.container.constraint()
+    constraint:set_strategy("max")
+    constraint:set_width(beautiful.notification_max_width or dpi(500))
+
+    rawset(constraint, "set_notification", function(_, notif)
+        constraint._private.notification = notif
+        local s = false
+
+        if notif.width and notif.width ~= beautiful.notification_max_width then
+            constraint.width = notif.width
+            s = true
+        end
+        if notif.height then
+            constraint.height = notif.height
+            s = true
+        end
+
+        constraint.strategy = s and "exact" or "max"
+    end)
+
+    return constraint
+end
+
+-- It is not worth doing a special widget for this either.
+local function notif_margins()
+    local margins = wibox.container.margin()
+    margins:set_margins(beautiful.notification_margin or 4)
+
+    rawset(margins, "set_notification", function(_, notif)
+        if notif.margin then
+            margins:set_margins(notif.margin)
+        end
+    end)
+
+    return margins
+end
+
+-- Used as a fallback when no widget_template is provided, emulate the legacy
+-- widget.
+return {
+    {
+        {
+            {
+                {
+                    wicon,
+                    {
+                        wtitle,
+                        wmessage,
+                        spacing = 4,
+                        layout  = wibox.layout.fixed.vertical,
+                    },
+                    fill_space = true,
+                    spacing    = 4,
+                    layout     = wibox.layout.fixed.horizontal,
+                },
+                actionlist,
+                spacing = 10,
+                layout  = wibox.layout.fixed.vertical,
+            },
+            widget = notif_margins,
+        },
+        id     = "background_role",
+        widget = wbg,
+    },
+    widget  = notif_size,
+}