Refactor shell completion scripts + add fish comletion

[folliehiyuki]

What this PR does:

• Add fish completion script
• Move shell completion scripts into 1 place scripts/shell-completion (they are renamed to the correct filenames on the user system)
• Add support to install completion scripts when running make install

NOTE:

• I'm not familiar with configure so the PR (apart from the fish completion script) is just me copying blindly what has been done with bash-completion.
• I tried adding fish completion for rules, partial rules and ids (from the output of usbguard list-devices and usbguard list-rules) but fish complains that there are too many arguments. Adding completion for the IDs alone seems pretty useless (with no indication of what the device with that ID is) so I give up on this topic.