Custom widgets support

A basic support for Custom widgets is provided since version 1.0.1. Designers can specify custom widgets within a Glade file, and for each custom widget they may specify a function name to be called to build it. The specified function will be searched and invoked among the View methods when the instance is created. View's method for custom widget creation has prototype:

 def func_name(self, str1, str2, int1, int2)

Creation functions are expected to return a widget object.