[python/viewer] Add customizable keyboard handling to Panda3d

[duburcqa]

Currently, keyboard handling is implemented in the environment itself, by intercepting key strokes on the terminal. It is working but confusing for the user since the window popup on top of the terminal and take the focus automatically, so that the user as to manually move the focus back on the terminal for keyboard handling to work. It is even more confusing that panda3d is already implementing a limiting keyboard handling, for example to close the window. It is not clear how to solve this issue though, because lambda functions needs to be executed, which is not working well with multiprocessing. One solution is to use a shared memory between panda3d process and main process, similar to what is currently done for handling keyboard in the terminal.