Implement inital file dialog

[fluxxcode]

This PR contains the initial implementation of the file dialog.

The following features are implemented:

• Select a file or a directory
• Save a file (Prompt user for a destination path)
• Create a new folder
• Navigation buttons to open the parent or previous directories
• Search for items in a directory
• Shortcut for user directories (Home, Documents, ...) and system disks
• Resizable window

Added the following examples:

• sandbox
• select_directory
• select_file
• save_file

Also added LICENSE and README.md.