added support for TECO stack tracing

 * when an error is thrown, stack frames are collected on clean up, up to
   the toplevel macro
 * the toplevel macro decides how to display the error
 * now errors in interactive and batch mode are displayed differently
   * in batch mode, a backtrace is displayed as a sequence of messages
 * Execute::file() forwards errors correctly
   * the correct error in the file is displayed in interactive mode
   * necessary to build the stack trace
String::get_coord() calculates line and column of a string position

 * use to get line and column into a stack frame
fixed formatting of "Text Editor and Corrector"
glib allocation functions throw std::bad_alloc exceptions now; catch all bad_allocs and convert them to State::Error

 * will allow some degree of OOM handling
 * currently does not work since the exception specifications prevent bad_allocs from propagating.
   exception specification usage must be completely revised
removed most exception specifications: allow bad_allocs to propagate

 * specifications resulted in runtime errors (unexpected exception) when bad_alloc ocurred
 * specs should be used scarcely: only when the errors that may be thrown are all known
   and for documentary purposes
fixed Execute::macro() and Execute::file() exceptions

 * might throw other exceptions that must be associated with the parent macro level's (stack frame)
 * add position information to "label not found" errors
 * Error copy constructors
use GLib's GError information to yield errors

 * results in better error messages, e.g. when opening files
 * the case that a file to be opened (EB) exists but is not readably is handled for the first time
added State::StdError class for constructing errors from std::exception objects
catch division by zero errors

 * C++ runtime does not automatically throw an exception
all interface classes define the same type InterfaceCurrent, simplifying the interface object definition

 * they still define their own classes (e.g. InterfaceNCurses),
   InterfaceCurrent is a typedef
report stack overflows as errors & fixed ValueStack dtor

 * ValueStack destruction might have resulted in Segfaults at shutdown
updated TODO