Shell|Unix: Cleaned up SIGWINCH handling, prevent Ctrl-C signal
Shell: Command history in the command line widget

Bash-like, editable command history.
Shell|libshell: Set up libshell as a sub-library of libdeng2

As a sub-library, libshell uses the namespace de::shell, and the
public header folder "de/shell".

The classes TextCanvas, TextWidget, and TextRootWidget were moved to
libshell so they can be used also in shell-gui and other executables.
Shell: Cleanup