Allows users to set an action to be triggered
when double clicking/middle-clicking on an empty space in the folder view. 
It will be super convenient, and improve user experience a lot!

Suggested default actions are "Ctrl+A" for double click, and "Ctrl+V" for middle click. In most of Linux DE middle click is set for pasting text by default, and it's not intuitive that the same doesn't happen in Dolphin when there's a file/folder in the clipboard.

https://phabricator.kde.org/D20532
Here this functionality have already been partially implemented, but Dolphin moderators rejected this suggestion, since they didn't think this functionality is important. 

But I'm sure this topic deserves further consideration. Many people will love to see this feature implemented, as it can be guessed even from this Reddit discussion:
https://www.reddit.com/r/kde/comments/de4sun/dolphin_are_you_interested_in_a_double_click_on/