Remove View::content property (and View::set() method)

[mvorisek]

improve #2075

View::set method was too abstract and clean override using LSP was not possible.

If you need View::$content property (and optionally set the data using constructor and/or View::set method), migrate to ViewWithData class.

There were two major usecases for View::$content property / View::set() method:

• set text data for Content tag
• set value for form control