control: define and export control_buildall(), control_cleanupall() to factorize

control: define and export control_buildall(), control_cleanupall() to factorize

control.c: put the definitions (from jackspa-cli.c) there:
- some changes, notably less pointer indirection

controls.h: define a controls_t type as control_t** and not as an array to make
it compatible with C++ as extern "C" (arguments that point to unbounded arrays
are invalid C++)

jackspa-cli.c: delete the definitions of build_controls() and
cleanup_controls() and use the new names

njackspa.c: rename build_controls(), cleanup_controls() to init_window(),
cleanup_windows() and call the new functions