Create the /completions directory and add a completion script for bash

Complete -i and -p with currently available players; complete the open
command with filenames

Create meson.build file in completions/ to install the completion script
to /etc/bash_completion.d; add completions subdir in root meson.build;
rename completions.bash to playerctl.bash

This reverts commit 5fe2e2e.

Shell completions should not be included in the build process. Instead,
package maintainers can decide how they want to distribute shell
completion scripts.

Rename bash completion script from 'completions.bash' to
'playerctl.bash' to make the file more easily identifiable oustide the
context of this repository