[*] General code improvement.

* Rework navigation allowing direct management and triggering for grabkeyboard widgets.
* Navigation code now behaves better with deeply nested layouts.
* Allow widget hierarchies deeper than 2 (__call() is implemented for every Widget metatable).
* Make BASE locals more secure (match '<filename>$' in regexp)

label.lua

@@ -1,4 +1,4 @@
-local BASE = (...):gsub('label', '')
+local BASE = (...):gsub('label$', '')
 
 local Widget = require(BASE..'widget')
 local core = require(BASE..'core')
@@ -7,7 +7,10 @@ local shadowtext = require 'lib.gear.shadowtext'
 local T = require('lib.moonspeak').translate
 
 -- Labels don't accept focus
-local Label = setmetatable({ nofocus = true }, Widget)
+local Label = setmetatable({
+    nofocus = true,
+    __call = function(cls, args) return cls.new(args) end
+}, Widget)
 Label.__index = Label