Shell improvements

Should wait for !476 (merged) to be merged before.

Improvements to the shell CLI:

• customizable bliss shell
  • configurable session setup
  • customizable status bar, prompt and style
• bliss prompt
• bind_key helper
• shell title
• provide access to REPL object in shell
• prdef function support for object instances
• prdef colored output
• show item units in scan header (if they are defined)

Fixes:

• setup_globals should be initialized before setup-file is loaded
• scan listener should take save flag from scan_info into account