[plug-in] Fix 'onDidSelectItem' behavior for Quick Pick widget

[RomanNikitenko]

I have the following code in my plugin:

const options = { placeHolder: 'Pick a machine to run the task' } as theia.QuickPickOptions;
            options.onDidSelectItem = (item => {
                // an item is selected
              });
theia.window.showQuickPick(items, options);

The problem is onDidSelectItem is invoked immediately after opening the Quick Pick widget.
At this step Quick Pick widget is in PREVIEW mode and first item gets selection by default.
So I think we should invoke onDidSelectItem when Quick Pick widget is in OPEN mode and user has ability to select some item.