Implement errorcontext_t

Function strerror(int errno) is replaced with
string_errorcontext(errorcontext_t * errcontext, int errno).

Also added static test "C-kern/test/static/threadsafe.sh"
which checks that strerror is not used in any module except
in single threaded tools.

Also implemented tool generrtab which reads all errors
with strerror and writes it in a table to STDOUT.

The project "projekte/pp-generrtab.prj" calls generrtab to
generate source file "C-kern/resource/generated/errtable.h"
which is included from "C-kern/context/errorcontext.c".
Comment functions init/free_glxwindow
Move event handler into dispatchevent_X11

The previous implementation used event handler registration
and dispatchevent_X11 called the registered handlers.

The new dispatchevent_X11 handles all events in a big switch
statement which puts all event logic into one place and is
therefore much more readable -- less sophisticated but more
readable.