Now the `App` contains a state which is a user-defined `struct` which
is passed to the `handle` and `contents` callbacks for `Container`'s and
`Element`'s. Built-in `Element`'s shall not access the `App.Model` and
should therefore never cause any side-effects.
User-defined events shall be used to act as *messages* to cause
potential side-effects for the model. This is the reason why only
the `handle` callback has a non-const pointer to the `App.Model`. The
`contents` callback can only access the `App.Model` read-only to use for
generating the *view* (in context of the elm architecture).
Renamed `Container.contents` to `Container.content` to be in line with
the corresponding `Element` function name. This has also been done for
the properties structs used by the `Container`.
This also means that currently the dynamic resizing through the app's
detached thread is not working, as it cannot send size updates. The
examples have been overhauled to still implement intermediate mode
applications accordingly.
Improve some examples to provide visual feedback, i.e. for the button
exmample, with fixes to make them compilable with the `Scrollable`
element changes.
