Add Qt Linguist standalone tool to the online installer

We offer Qt Linguist as a standalone tool, as creating translations does not require installation of the whole Qt.

Let's add Qt linguist to Qt Online Installer as well. It should be available similarly to design tools, which can be installed and used without installing all Qt libraries as well. This would also improve Qt for MCU developer journey, as Qt for MCU users would benefit installing Qt Linguist from the same place, where they get other MCU tools and libraries. However, still they have no need to install whole Qt.