#1392: Smart completions

[HenokLachmann]

This PR fixes TodoIssueID #1392

Implemented changes:

Auto-completion is extended for the Mvn commandlet. It enables autocompletion for the ShellCommandlet as well as for bash. Completion is supported for the last word as well as intermittent words on a commandline. Completion covers the following:

• Typical maven goals like clean, install, package
• Boolean flags like --fail-fast
• Definitions like -Dexec.mainClass=<classname>

Whitespace is excluded after a completed trailing = by differentiating partial from full completions.

A bug existed in the current implementation that would complete the exit subcommand infinitely. This is fixed by this PR.

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