Using a hack from Dino:
https://github.com/dino/dino/blob/v0.2.0/main/src/ui/chat_input/chat_text_view.vala#L51

"Entry" is a widget in GTK and "input" is something more generalized.

With an eventually upcoming file attachment button (and separate voice
button) the UI might become too clunky with a send button. In addition,
no other major desktop chatting program seems to have one, since sending
via pressing enter is the standard.

Basically reverts 6b9f691c

Crediting Dino.im for their work.

What's still needed is to only activate the scrollbar of the input when
it's needed. Otherwise the input gets too big.
https://github.com/dino/dino/blob/231df1/main/src/ui/chat_input/chat_text_view.vala#L51
Related to #71

Related to #71

Fixes #70

I think the Model-View-Presenter pattern is better for this than the
Model-View-Controller pattern.

The name "controller" is used in the MVC pattern context here, so we
should rather go with another name.

I decided to go with the MVC (Model-View-Controller) pattern. This is
the first step by making MainWindowContainer a MainWindowView.

Although it isn't necessary for Python 3, some tools analyzing code
(e.g. pyreverse from pylint) need those files.