Fix IME doesn't work under Popup on Windows.

[zaevi]

Fix #77837 and fix #73591

Set keyboard focus for is_popup window otherwise it won't produce WM_IME_STARTCOMPOSITION and IME won't work.

See https://learn.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-setfocus:

If a window is active but does not have the focus, any key pressed produces the WM_SYSCHAR, WM_SYSKEYDOWN, or WM_SYSKEYUP message. If the VK_MENU key is also pressed, bit 30 of the lParam parameter of the message is set. Otherwise, the messages produced do not have this bit set.

[akien-mga]

#73591 mentions the same issue on macOS so I guess it wouldn't be fully fixed by this PR.

[zaevi]

#73591 mentions the same issue on macOS so I guess it wouldn't be fully fixed by this PR.

I don't have macOS env to debug, but the cause might be similar.

[bruvzg]

The same was done for Linux/X11 - #72497

I can reproduce it on macOS, but not on Windows, but in general a window that is expected to take keyboard input should have keyboard focus.

[akien-mga]

Thanks!