input_method: Split up update method

[Nefsen402]

The update method was a big hammer, it did a lot of things.

• This function would recreate the scene every time the input method client commits a new buffer

Instead, we create the scene when the input method surface maps / unmaps. When the input method is unfocused, we simply disable the scene node.

Fixes: #8086

[emersion]

Gentle ping :)