See #469

* making userName_ an AbstractButton should make it play nicely with the ListView

* make Avatar an AbstractButton

* make userName_ in Reply.qml an AbstractButton

* use alias for avatar background color and fix margin of username in reply

It seems that you need to manually destroy created objects... Great...

fixes #898

* add sendernames to state events

* Center state events and make them less tiny

That way we can autohide the scollbar if needed, it should fix some
jumping issues, it makes it possible to flick on mobile, etc.

Some related bugs:

https://bugreports.qt.io/browse/QTBUG-75223
https://bugreports.qt.io/browse/QTBUG-44902

Rows in the message bubble design are only as wide as the bubbles. This
lead to invisible hover highlight. Now it's consistent.

fixes #941