Add support for JSX Fragments

[tbuschto]

Currently the shortest syntax is to add multiple JSX elements in one append call is to use commas:

ui.contentView.append(
  <button centerX={0} top={100} text='Show Message'/>,
  <textView centerX={0} top='prev() 50' font='24px'/>
);

Very ugly. The alternative is to use WidgetCollection:

ui.contentView.append(
  <widgetCollection>
    <button centerX={0} top={100} text='Show Message'/>
    <textView centerX={0} top='prev() 50' font='24px'/>
  </widgetCollection>
);

However, this is much longer. In React there is also the fragment element, which would make it look like this:

ui.contentView.append(
  <>
    <button centerX={0} top={100} text='Show Message'/>
    <textView centerX={0} top='prev() 50' font='24px'/>
  </>
);

TypeScript added support for this syntax in 2.7, but it does not work in Tabris.js because we use the jsxFactory compiler option. This is a limitation that may be eliminated in the near future: microsoft/TypeScript#20469

Once fragments work with jsxFactory we should support it.

[tbuschto]

Compromise: #1679