Missing sway visible attribute

[g33kex]

In sway's tree there is a "visible" attribute that tells whether a node is visible or not : https://man.archlinux.org/man/sway-ipc.7.en#4._GET_TREE

The "visible" attribute is present in i3ipc documentation, but is not implemented:

i3ipc-python/i3ipc/con.py

Lines 94 to 98 in 14b5636

	ipc_properties = [
	'border', 'current_border_width', 'floating', 'focus', 'focused', 'fullscreen_mode',
	'id', 'layout', 'marks', 'name', 'num', 'orientation', 'percent', 'scratchpad_state',
	'shell', 'sticky', 'type', 'urgent', 'window', 'pid', 'app_id', 'representation'
	]

[acrisci]

You can go ahead and add that if you want.