Application shell widgets must take focus on activation

[spoenemann]

When a widget is activated, its onActivateRequest method is invoked. However, the default implementation does nothing! For widgets that are added directly to the application shell, this is a problem, because ApplicationShell tracks the active widget through a FocusTracker, which listens to focus changes. So if a widget that is added to the shell does not implement onActivateRequest, it will never be seen as active. This causes peculiar side effects; for example the tab bar context menu commands operate on the current (last active) widget, so doing right-click and Close on a widget's tab can cause the wrong tab to close (the last one that took focus instead of the one that was right-clicked).

I fixed this problem for the already implemented views in #1028, but it remains an issue for every new Theia widget. It is not obvious for widget implementers that they need to take care of the focus.

[spoenemann]

One possible solution could be to create a common abstract superclass for all application shell widgets that has some default behavior regarding focus, e.g.

    protected onActivateRequest(msg: Message): void {
        this.node.focus();
    }

However, this works only if the node of the widget can take focus. HTML elements that do not have any default user interaction, such as div, are focusable only if the tabindex attribute is set.

[svenefftinge]

Instead of solving it generically, which seems to be hard. We could maybe improve the error reporting for widgets, that didn't get focus.