Add an option to disable inline song metadata editing through click

It's too easy to edit song metadata by mistake simply by clicking on the tune, and it's been annoying me lately . Some albums in my library are also loaded in a peer-to-peer client, and modifying the on-disk files would mean "corrupting" them.
This commit adds an option that allows the user to disable inline editing through simple click.
Looks like this is not used, as you read s.value("click_edit_inline") directly in playlistview.cpp (which seems fine, so I think you can simply remove this line)