logical program structure extensions proposal for kivy language

kv lang extensions proposal

# proposal for kivy language extensions

BoxLayout:

    ############
    # conditions

    if root.foo:
        Label:
            text: 'Condition true'
    elif root.bar:
        Label:
            text: 'Other condition true'
    else:
        Label:
            text: 'Condition false'

    #######
    # loops

    for ob in self.iterables:
        Label:
            text: ob.something_from_ob

    ############
    # exceptions

    try:
        Label:
            text: root.computed_label
    except AttributeError as e:
        Label:
            text: 'Cannot compute label: {}'.format(e)