Unify deprecated 'Property.value' handling

[mpsonntag]

The Property.value attribute is deprecated!

• the Property.value accessor methods currently use a print statement. Refactor to a proper deprecation warning using the warnings module.
• in Property.__init__ the value can be keyword set, but does not elicit the deprecation warning.
• in the Section.create_property() method value can be set as a keyword argument, but values can not. Refactor so that values can also be keyword set and that the usage of the value keyword elicits a deprecation warning.