Fixes a timing issue where dialogs got configured right before having
their parent set. This gave compositors the time to resize/rearrange the
dialog as if it were a normal window, only to be marked as "floating"
right away. On niri, this manifested as huge dialog windows.
This is achieved with the addition of a `p_parent_id` parameter to
`window_create`, akin to its popup counterpart.
`window_create` now also accepts a single `Size2i` parameter instead of
two integers, in line with the rest of the `WaylandThread` API. The
original idea was to have a very "barebones" API, akin to the `drivers/`
directory, but that didn't pan out.
4df96fc7ab