Use event.currentTarget and not this in handlers

[ivmartel]

Some frameworks seem to run event handlers in a different context than the one used in web pages. This changes the value of this. To make them safer, event handlers on the application should use the event parameter and its currentTarget to replace this (to which it usually points to as written in the HTML5 recommendations the-event-handler-processing-algorithm).

Inferred for Vue in Instance-Lifecycle-Hooks: this refers to the Vue instance.

Inferred for Angular in user-input; this refers to the component class.

Relates to #363 (and similar...).