Draft: feat: Miller Column View

So, this was a lot harder than I thought. What a surprise 😄

In my view this is a MVP for miller column and the basis for future improvements. It is not a exact copy of apples finder but has its own KDE touch, e.g. there is no default active preview on the rightmost column, but with F11 and the feature-full preview window you get a even better workflow.

It is still (a bit) rough around the edges, bugs and areas of improvement are to be expected.

Links

• Fixes https://bugs.kde.org/show_bug.cgi?id=387455
• Interest in: https://discuss.kde.org/t/dolphin-column-view/7986
• Interest in: https://discuss.kde.org/t/miller-columns-in/32797