Fix the initial candidate window position

[rhysd]

Fixes #2792

The issue is that the candidate window is already open when InputMethod::Open event is handled in the window manager. Since set_ime_cursor_area is not called yet when the window is opened, the position of the window is at the top left corner.

To fix this, we need to call set_ime_cursor_area earlier before the window is opened. winit's example calls the function just after calling set_ime_allowed.

This PR calls the function when allowing IME so InputMethod::Allowed event now has the position of the window.

Here is the video:

fix.mp4

[kenz-gelsoft]

I think this is sane fix for the problem I catched earlier on original PR:

#2777 (comment)

I investigated winit's macOS platform code and I find it correctly updates the internal ime_position queried from the OS and invalidates it when we call winit::Window::set_ime_cursor_area().

And as far as I tested macOS iuput methods doesn't reposition immediately while it is currently displayed. This is not only on winit apps but also on native macOS apps.

So this is not a bug of winit but is a cross platform limitation. We need to set_ime_cursor_area() before the subsequent Preedit event I guess.

Sorry about the misinformation on the decision of the initial API design.