[RFC] Implement wibox.widget.template for the awful.widget.layoutbox

[Aire-One]

Hello,

With my other PR (#3421) being ready for review, here is the implementation of the template widget for the awful.widget.layoutbox. This branch is based on the other PR, only the last commit (eba12a0) is relevant.

With the current implementation, an end-user can use/copypast this code example in the awesomerc.lua to customize the layoutbox:

    s.mylayoutbox = awful.widget.layoutbox {
        screen  = s,
        buttons = {
            awful.button({ }, 1, function () awful.layout.inc( 1) end),
            awful.button({ }, 3, function () awful.layout.inc(-1) end),
            awful.button({ }, 4, function () awful.layout.inc(-1) end),
            awful.button({ }, 5, function () awful.layout.inc( 1) end),
        },
        widget_template = {
            template = wibox.widget.textbox,
            update_callback = function(widget_template, args)
                local layout_name = args and args.screen and awful.layout.getname(awful.layout.get(args.screen)) or "dunno"

                widget_template.widget.text = "custom layoutbox : " .. layout_name
            end,
        },
    }

I open this PR in draft mode, waiting for the other one to be merged. After the merge, I'll rebase this branch, and it'll be ready.