Add bash completion script (#1460)

* Add bash completion script

* Move completion script to shared

* Remove shebang from bash completion script

Completion scripts are sourced not executed.

* Add bash completion script to CMakeLists.txt

* Add completion for MIME types

* Add completion for sessions

* Guard commands that need a running CopyQ server

* Refactor bash completion script

Use an array for command names, move common completion logic into
functions, and remove unused patterns from the case statements.