Call native window methods from NiceGUI main thread

[rodja]

This PR provides a pywebview proxy object app.native.window to call methods on the window object in the pywebview process. This solves #909 and other issues like #770. It may also be used as basis to implement a native file picker. Possibly the core requirement behind #892 is also related.

Example usage:

async def startup():
    app.native.main_window.resize(1000, 700)
    app.native.main_window.load_url('https://nicegui.io')
    w, h = await app.native.main_window.get_size()
    x, y = await app.native.main_window.get_position()
    print(f'window: {w}x{h} at {x},{y}', flush=True)
    await asyncio.sleep(1)
    selected_files = await app.native.main_window.create_file_dialog()
    print(selected_files, flush=True)

async def update():
    on_top.set_text(f'{await app.native.main_window.is_always_on_top()}')

ui.switch('ontop', on_change=lambda args: app.native.main_window.set_always_on_top(args.value))
on_top = ui.label()
ui.timer(0.1, update)

app.on_startup(startup)

ToDos

• replace print messages with logging
• allow receiving of return values from window methods (awaitable?)
• implement get_size and get_position
• provide type annotations
• documentation

[retsyo]

anyway to set the caption of native file dialog? for example, "please select config file to load"

btw, app.native.main_window.is_always_on_top() is app.native.main_window.get_always_on_top() in current