Add zsh completions for plasmoidviewer

Supports all the (non-generic-Qt) command line options, provides
meaningful completion contexts, completes applets, containments and
themes from lists of actually packages, as well as plain fixed lists
of choices like location and form-factor.

Note: it uses the new variable KDE_INSTALL_ZSHAUTOCOMPLETEDIR, which was
added in ECM 5.101.

Read more: frameworks/extra-cmake-modules!310