Allow using ui.menu for custom context menus

[falkoschindler]

Description

On Reddit we've been asked how to create a custom context menu with NiceGUI. Theoretically, it should be possible to use QMenu's context-menu and touch-position props:

with ui.image('https://picsum.photos/id/377/640/360').classes('w-96'):
    with ui.menu().props('context-menu touch-position'):
        ui.menu_item('Copy')
        ui.menu_item('Delete')

But this does not work for two reasons:

• We need to activate the parent event (see Remove no-parent-event prop for ui.menu #713). As a workaround we can remove the no-parent-event prop: ui.menu().props(..., remove='no-parent-event').
• We need to deactivate the loopback. As a workaround we can write ui.menu.LOOPBACK = False.

To simplify the API, we want to

• Remove no-parent-event prop for ui.menu #713,
• deactivate loopback for ui.menu, and
• add a context menu demo for menu documentation.

As an alternative workaround one can simply use ui.element('q-menu') (without value events and binding):

with ui.image('https://picsum.photos/id/377/640/360').classes('w-96'):
    with ui.element('q-menu').props('context-menu touch-position'):
        ui.menu_item('Copy')
        ui.menu_item('Delete')