Add Gnu readline for use in shell mode.
This patch make using the shell in interactive mode a lot easier!
Features:
- Auto-completion of actions (at start of line) and filenames.
- History (up/down arrows).
- Command line editing.
Note:
- Gnu Readline is GPL 3 (not LGPL3). Inkscape is nominally GPL 2.
- Detection of system support for readline should be tested (seems to work on Linux and OSX).
This replaces and earlier merge request for a borked branch.