When creating a window, Godot would first register it to the WM(show it) and then set its flags.
This works fine on a floating WM, but on tiling WMs as soon as a window gets registered
the WM immediately acts on the window by scaling it up and treating it as a generic window,
being registered without any special flags.
This commit separates the showing of the window into another function and calls it after the most important flags are set,
making windows with special flags(eg. all popups) work again on tiling WMs.
Fixes#37930
d670a49612