Implement tooltip level on Windows platform

[djeedai]

Implement WindowLevel::Tooltip on Windows platform.

Before change:

• Tooltip window appear in taskbar
• Window has strange min-size (probably due to OS min constraints)
• Window steals focus from main app window (see grayed out title bar of main app)

After change:

• Tooltip window doesn't appear in taskbar
• Window fits any size (tested with nursery's tooltip.rs)
• Window doesn't steal focus from main app window

Details

The change adds SWP_NOACTIVATE in many calls to ShowWindow() to prevent unwanted Win32 activation stealing the focus. SWP_NOOWNERZORDER is also added, inspired by Qt5 (see here).

Some is_focusable boolean is added to the state to discriminate between a regular "restore" and a non-activating show when ShowWindow() is called, because the former implicitly activates the window, stealing focus from the main app window and giving it to the tooltip.

Tentative WS_ and WS_EX_ flags are set for WindowLevel::Modal and WindowLevel::DropDown. They have not been tested, but feel like a good starting point for whoever's coming next to implement those. In particular note that DropDown almost surely wants to be WS_CHILD to allow keyboard focus while not graying out the main app window title bar. However, because they have not been tested, WindowBuilder::set_level() continues to emit a warning for those levels, and ignore them for now for consistency with the current behavior.

[arthmis]

Ok where did you find how to make the tooltip not steal focus? I was searching on how to do that but I came up empty. Where did you find about the calls to ShowWindow() stealing focus? And thank you for doing this!

[arthmis]

I'm putting this here because this pr addresses some issues from this issue #1626

[djeedai]

Many trial and error with most WS_ and WS_EX_, followed by some what-would-qt5-do look at QWidget source code. Eventually found the right combination. 😏

[jneem]

I'd be happier if someone who knows windows took a look, but I think it's been sitting long enough and it looks pretty much fine to me.

[djeedai]

Thanks! 👍