Make PopupMenu hide after its parent window loss focus

[Nikitf777]

Previously PopupMenus were still on screen after their parent was minimized. It was quite annoying, especially in Editor, because it limits its fps to 1 per second when minimized, even if one of the popup menus is still visible. It's also worth mentioning that no other app I use behaves like this. Their popups always hide after minimizing the main window or they can't just be minimized while popup is visible.

Before

Screencast.from.2024-10-16.21-09-35.mp4

After

Screencast.from.2024-10-16.21-11-24.mp4

I decided to fix this by adding two flags which are also visible from scripts:

• Window::return_to_transient (default: false)
• PopupMenu::hide_on_parent_unfocused (default: true)

Also I had to add auxiliary const method Window::get_transient_parent() to be able to use the "focus_exited" signal of the parent window from its child popup.

Previous behavior can be achieved by setting return_to_transient to true and hide_on_parent_unfocused to false.

Three things I'm not sure about are:

• naming of the PopupMenu::hide_on_parent_unfocused and related stuff (I chose this name because there is Popup::_parent_focused() method.
• using thread guards in setter and getter of the Window::return_to_transient flag. Honestly, I don't even know what exactly do ERR_MAIN_THREAD_GUARD and ERR_READ_THREAD_GUARD_V macros do
• Haven't tested on .NET version of Godot.

[KoBeWi]

Can't reproduce the bug on Windows.

[Nikitf777]

Can't reproduce the bug on Windows.

Yep, it seems to be a Linux only issue (and maybe macOS), but it needs to be fixed somehow. Looks like it's due to how different OSes handle popups. And I don't think it'll be changed by them in the future. These are different approaches.

[bruvzg]

• The issue is not reproducible on Window and macOS, but both work fine with this change.
• The issue is reproducible and fixed on Linux.

Please update documentation for new functions, see Contributing to the class reference and squash the commits, see Pull request workflow.