Merge: Abstract attribute

Add the annotation `abstract` on attributes.

It is just a syntactic sugar to define a couple of abstract getter-setters with a shared documentation without an associated slot in the instance so it can be used in interfaces.

~~~nit
interface Foo
	var a: Object is abstract
end

class Bar
	super Foo
         # A concrete attribute that redefine the abstract one
	redef var a
end

class Baz
	super Foo
	var real_a: Object
         # A pair of concrete methods that redefine the abstract attribute
	redef fun a do return real_a
	redef fun a=(x) do real_a = x
end
~~~

The visibility rules are unchanged with regard to concrete attributes, so by default the writer is private.

Needed cleaning (and a bugfix) are included in the PR

Pull-Request: #1177
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>

diff --cc src/compiler/abstract_compiler.nit
Simple merge

diff --cc src/modelize/modelize_property.nit
Simple merge

diff --cc src/rapid_type_analysis.nit
Simple merge