Provide decorator for alias_property

[ikus060]

Is your feature request related to a problem? Please describe.
Defining AliasProperty explicitly with getter and setter is a bit painful and not very clear as we need to define 2 function and then create the AliasProperty.

Describe the solution you'd like
I'm proposing to kivy to provide alias_property decorator to easily create the alias property.

Current implementation

    def _get_disk_total_text(self):
        total = self.disk_usage.get('total', 0)
        humanfriendly.format_size(total)
        if total <= 0:
            return ""
        return _('Total space %s') % humanfriendly.format_size(total, binary=1)

    disk_total_text = AliasProperty(_get_disk_total_text, None, bind=['disk_usage'])

New implementation:

@alias_property(bind=['disk_usage']
def disk_total_text():
        total = self.disk_usage.get('total', 0)
        humanfriendly.format_size(total)
        if total <= 0:
            return ""
        return _('Total space %s') % humanfriendly.format_size(total, binary=1)

It's clean and concise.

A basic implementation supporting getter would look like this:

def alias_property(bind=[]):
    """
    Alias property decorator.
    """
    return functools.partial(AliasProperty, setter=None, bind=bind)