add example dialog which provides interactive commands

[kkeybbs]

Example tests/dialog.c is useful.

• Contains all dialogs ( from other examples ).
• Provides interactive commands open/opens/save/browser/exit.
• Can be used as a cross-platform prebuilt binary tool, to help command line tools interactive with open/save/browser-folder dialogs.
• Use it like this:

# open multiple files
echo $'opens\nexit' | ./dialog

# interactive
./dialog
\> open     # 'open dialog' will be shown.
\> save     # 'save dialog' will be shown.
\> exit     #  exit interactive program.

[mlabbe]

Cool idea, but not core to the library or something that I want to maintain going forward. If you create it as a separate repo, let me know and I will link to it.