Subwindow issues on Windows

Nobody has claimed this yet.

This issue will have to wait on a few enhancements to druid.

One issue is that `WindowLevel` is not implemented for Windows 10. Creating subwindows on Windows 10 uses the `platform::WindowBuilder::build` function that is used to create app level windows. The issue is that this function treats all windows as [overlapped windows](https://docs.microsoft.com/en-us/windows/win32/winmsg/window-features#overlapped-windows). As a result dropdowns and other subwindows show up on the taskbar.

For modals, tooltips, and dropdowns they should be [pop up windows](https://docs.microsoft.com/en-us/windows/win32/winmsg/window-features#pop-up-windows) I believe.

The last issue is creating a subwindow takes focus away the parent window which is fine depending on the type of subwindow. For something like a tooltip though, it shouldn't take away focus.

A small fix to make the popup windows not show up in the taskbar is to set their extended style to `WS_EX_TOOLWINDOW`. The other fix would be to wait on the work being done to embed child windows within an App window. On Windows 10, setting the parent handle for a popup window prevents the popup window from showing up on the taskbar.

Start at the Windows implementation of platform::WindowBuilder::build and review how subwindows are created for modals, tooltips, and dropdowns. Check the Druid enhancements mentioned in the issue before changing behavior. Done means Windows subwindows use appropriate popup/taskbar behavior and tooltips do not incorrectly take focus.

Written by the indexing model from the issue text.