FilePicker and uploads

[FeodorFitsner]

New methods and properties:

• page.get_upload_url(file_name, expires)
• page.overlay: List[Control]

New attribute in flet.app():

flet.app(target=main, upload_dir="uploads")

Fletd environment variable: FLET_UPLOAD_ROOT_DIR

New FilePicker control:

• dialog_title: str
• initial_directory: str
• file_name
• file_type
• allowed_extensions
• allow_multiple
• result: FilePickerResultEvent
• on_result
• on_upload (FilePickerUploadEvent)
• pick_files(dialog_title, initial_directory, file_type, allowed_extensions, allow_multiple)
• save_file(dialog_title, file_name, initial_directory, file_type, allowed_extensions)
• get_directory_path(dialog_title, initial_directory)
• upload(files: List[FilePickerUploadFile])

FilePickerUploadFile class:

• name
• upload_url
• method (PUT (default), POST)

FilePickerUploadEvent class:

• file_name
• progress
• error

FilePickerResultEvent class:

• path
• files: List[FilePickerFile]

FilePickerFile class:

• name: str
• path
• size