Moved standard tk and ttk widget classes to stdwidgets.py.
Moved pygubu custom and utility widgets to pygubu.widgets module.
Better widget tree selector on ui builder.
Fix some examples
Clear properties editor on New and Open menu actions.
Setup properly read-only properties when creating widget.
Add validator for validate entry command.
Don't show values property for ttk.Entry.
Delete previows value when setting 'text' property on ttk.Entry
Add from_string method.
Add some tests.