Shell needs a way to figure out if it should show the keyboard button
Although i'm not 100% sure this will work out we should follow @tobias.bernard idea to only show the show/hide keyboard button for clients where only virtual-keyboard-unstable-v1
is in use. Therefore the shell needs a way to infer that from the OSK (similar to the visible
attribute.
This way we can hide the keyboard button when more elaborate tings like text-input/input-method are in with a particular client.