Support for modal windows

[Enyium]

Context: Rust.

I propose to add a new constructor function to generated Window/Dialog component structs besides new(): new_modal() (with_owner() would be a terminology that's too Windows-specific). It would take a reference to another type implementing ComponentHandle as parameter. Because of lack of support from winit, on other OSs than Windows, this currently simply would be ineffective. On Windows, winit provides WindowAttributesExtWindows::with_owner_window(), which you'd need to use. Whenever the owned window is shown, the owner window must be disabled (with WindowExtWindows::set_enable()). When the owned window is hidden, the owner window must be enabled before hiding it.