Some other changes to the CMake-files done as well:
* documentation looks more like Doxygen-style
* prevent mix-up with C++ GSL library ("Guidelines support library" not
"GNU scientific library")
* using `get_wch()` now and mapping most of the function keys documented
in `vterm_keycodes.h` (not num keypad)
* there are still a lot of mappings from ncurses missing (in conjunction
with modifier keys, related issue #11)
* using mutex to make sure writing to terminal client is serialized
Christian Burger