added a stub for a python-based Mode
big commit: tweaks to edit_mode, added skeleton for search mode, and fixed some compile errors found by clang
Lots of very tricky work on this: Got a proper varargs_t object working incl. interop with python (nightmares)
Had to implement a special "callable_t" object for wrapping func_ptr/usr_data in C and PyObject for python interop.
Implemented a mode stack in screen.

After all the pain, we can now trigger any mode in python in a stack-fasion. Python can provide python-based callbacks to native C modes.