Make Focus() / Blur() reliable and let Blur() return focus to the previous app

[lijy91]

Window::Focus() and Blur() follow the platforms' simplest calls and fail in common cases:

• Windows: Focus() is SetForegroundWindow + SetFocus, which the foreground lock refuses when the app isn't in the foreground (only the taskbar flashes). Consider the AttachThreadInput / AllowSetForegroundWindow approaches, and check IsFocused() after focus changes.
• Linux: gtk_window_present() without a timestamp is subject to focus-stealing prevention; use gtk_window_present_with_time() with the last event time.
• Blur() should hand focus back to the previously active window or app. On macOS it only does orderBack:, and on Windows SetFocus(nullptr), which leaves the app active. Typical use: a launcher/search bar that hides and pastes into the previous app. Related: libnativeapi/nativeapi#59 (app-level hide on macOS).

Background:

• setFocus not always work and isFocused is always false. leanflutter/window_manager#364: focus() not always working, isFocused() always false (Windows)
• [Linux] WindowManager.focus() does not focus the window. Ubuntu. leanflutter/window_manager#218: focus() doesn't focus the window on Ubuntu
• How to unfocus window? leanflutter/window_manager#387: how to unfocus the window and bring back the previous one
• Is there any way to focus on the previous window when I call windowManager.hide()? leanflutter/window_manager#352: focus the previous window when hiding