[Approved (!)] Plasma 6 proposal: double-click to open files and folders by default

This was discussed and agreed to at Akademy 2022, but not everyone was there, so I'm opening this issue here to make sure all relevant voices are heard.

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This debate has been going on for years, and basically boils down to the following positions:

"Single-click is objectively better and more internally consistent for opening items, and those are more important than any concerns about familiarity for migrating users or usability when selecting items."

"Double-click is more familiar to people migrating from other systems and has objectively better usability for selecting items, and those are more important than what's better or more internally consistent for opening items."

FIGHT!

...or not, because over time, distros have been making the decision for us. At the moment, Kubuntu, Fedora KDE, and Manjaro all default to double-click, and these are major distros. Many of the major distros that do not change this setting follow upstream settings (e.g. Arch, Neon, Debian) so not switching doesn't imply satisfaction with the status quo, just a preference for or policy of not changing upstream settings. A couple of other smaller distros also default to double-click, such as NetRunner and FerenOS.

Distros are close to users and clearly the feedback they've been getting is that double-click is a better default.

Let's admit it and switch to double-click by default ourselves.