Draft: Ensure the application window actually fits on the screen

Prior to this patch, the initial size of a Kirigami application might end up wider or taller than the available screen realestate. The sizes are fairly small, but also we cannot simply assume that these are always true. So, clamping to the actual available size ensures that we don't try and paint a window that's too wide/tall.

Further, this introduces API to request an initial size of the window, which will then replace the default width/height values (and it will still be clamped).