Document how to work with files on desktop

[AlexeyTsvetkov]

Being able to open a file (via a dialog or drag & drop) is essential for many desktop apps.

While the project does not provide relevant widgets, there are known solutions:

• java.awt.FileDialog or javax.swing.JFileChooser can be used as a file dialog;
• Since 1.4.0 it became possible to implement a drag & drop widget easily by using onExternalDrag API (Will support drag and drop file into widget? #222);

However, there is no easily discoverable tutorial or example showcasing these solutions.

Questions regarding working with files have been asked and commented many times:

• Call system Explorer/Finder to select files in Compose #176;
• File chooser support #197;
• Will support drag and drop file into widget? #222;
• How to use a FileChooser() in Compose.  #1003;
• Native support for File Operations #3294;

The issues above contain answers, but searching for documentation in issue tracker is suboptimal.
It would be better to have a tutorial or a simple example, showcasing how to use AWT/Swing file dialogs and onExternalDrag.

[okushnikov]

Please check the following ticket on YouTrack for follow-ups to this issue. GitHub issues will be closed in the coming weeks.