Draft: Change from single- to doubleclick as default

As suggested in https://phabricator.kde.org/T8187#245157, I am creating a new MR for this (old one was !121 (closed)).

This follows some controversal discussion and I am rather sure there are still many people who don't like this change. This is not thought to make you upset. I feel however, that the arguments presented in the discussion currently favor double click. Feel free to challange this opinion (ideally here) or add further arguments (ideally in the Phabricator task, also make sure the argument has not already been brought up, there is a summary in the description).

This is still untested and probably requires further changes to be sure it works and also does only affect new installations.

Currently I am not sure whether changing the default in .kcfg files will also change the value for existing users. The relevant API I found for this is a bit abiguous about this, at least with my limited knowledge:

An entry can optionally have a default value which is used as default when the value isn't specified in any config file.

It has been mentioned that KConfig update scripts might be needed: !121 (comment 92478).

Edit: I now created a first draft of an update script approach (not tested and that might still need alot of additional work).

To be merged together with plasma-integration!11 (closed).