Add the ability to toggle ImGuiViewportFlags_TopMost after viewport creation on win32

[Kyuusokuna]

Currently the example imgui_impl_win32 does not respond to changes of ImGuiViewportFlags_TopMost as the SWP_NOZORDER uFlags and NULL hWndInsertAfter passed to SetWindowPos() prevents a window from changing its topmost style.

According to https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-setwindowpos hWndInsertAfter must be either HWND_TOPMOST or HWND_NOTOPMOST and uFlags must not contain SWP_NOZORDER in order to change the topmost style.

Testcase:

static bool topmost = true;
ImGui::Begin("Topmost Test");
ImGui::Checkbox("Topmost", &topmost);

if (topmost)
    ImGui::GetWindowViewport()->Flags |= ImGuiViewportFlags_TopMost;
else
    ImGui::GetWindowViewport()->Flags &= ~ImGuiViewportFlags_TopMost;

ImGui::End();

[ocornut]

Thank you very much Sven, I appreciate the patch and details/links. Merged now!