x11-toolkits/nanogui: Fix several issues

• nanogui doesn't depend on Eigen any more
• use external glfw package
• for Python, __init__.py is installed
• for tests, zenity executable is installed

PR: 263194
Original patch submitted by: Hiroo Ono <hiroo.ono+freebsd@gmail.com>

(cherry picked from commit 563c4a8cd859eeb4657a6d54730d65bde1541d36)