Avatar: fix code style, port actions to QObjects
Fix up code style, optimize some code
Use modern C++ stuff for loops; reset QML Control paddings instead of rebinding manually in a less efficient JavaScript.
Port actions group property members to QObject* and manage lifecycle
It is important to emit change signals whenever a QObject member is destroyed. MEMBER syntax of Q_PROPERTY won't manage that for you.
Also, main and secondary actions were meant to be Kirigami.Actions, but
since Kirigami.Action is a pure QML type and QQC2.Action is in a
private header, QObject* is unfortunately as close as we can get to its
real type. Which is still better than QVariant, and gives us some
stronger nullability guarantees (e.g. can't be undefined or other
meaningless type).
Note that as discussed in #50 (closed) actions might get removed soon. But even in that case, I'd prefer removing some working code.