[*] 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)

columns.lua

@@ -1,21 +1,20 @@
-local BASE = (...):gsub('columns', '')
+local BASE = (...):gsub('columns$', '')
 
 local Layout = require(BASE..'layout')
 
-
 -- Advance position to next column,
 -- given current position, widget dimensions and padding.
 local function columnadvance(x,y, ww,wh, padding)
     return x + ww + padding, y
 end
 
-function Columns(args)
-    local self = Layout.new(args)
+local Columns = setmetatable({
+    advance = columnadvance,
+    __call = function(cls, args) return cls.new(args) end
+}, Layout)
+Columns.__index = Columns
 
-    self.advance = columnadvance
-    self.prev = 'left'
-    self.next = 'right'
-    return self
-end
+
+function Columns.new(args) return setmetatable(Layout.new(args), Columns) end
 
 return Columns