[*] 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)

core.lua

@@ -1,4 +1,4 @@
-local BASE = (...):gsub('core', '')
+local BASE = (...):gsub('core$', '')
 
 local core = { theme = require(BASE..'theme') }
 core.__index = core