Selection issues introduced due to mixed-mode #1728

Problem

Single click navigation leads to a few different issues and annoyances relating to selection of folders:

1. In list and column view, if the window is full of folders and all of them are selected, it becomes impossible to deselect them using the mouse. If only some of them are selected, trying to deselect them by clicking on a different folder opens that folder.
2. In column view after selecting multiple folders, if a selected file is mis-clicked, all of the selected folders are opened in new tabs. This is especially problematic when dealing with a large number of folders.
3. In the other views, this does not happen. However It is unclear how to open multiple selected folders (single clicking them doesn't sound like a good idea, as explained in point 3).
4. Often I will select a file/folder with my mouse, press ctrl+x/c, navigate to a different window/tab using the mouse and press ctrl+v to paste, or use a keyboard shortcut to rename the folder etc. Using the mouse and keyboard shortcuts in combination for quick actions has become much more cumbersome now.
5. When scrolling with the mouse, if I choose to start scrolling with the keyboard, I must start from the beginning, since that is the current "selected" file. I could use the blue select icon to select a folder and resume scrolling from there but if I wish to select multiple folders, I will have to scroll to the very top and deselect the first file/folder. Plus it is very easy to miss the blue icon and click the folder (and open it) instead, especially if the icon sizes are set to low in list/column view.
6. When navigating through folders and arriving at the target file, it doesn't always register in my mind to double click it (especially in column view).

Proposal

Problem number 3. could be solved by opening selected folders using double click, while in mixed mode but this is something that will always be confusing.

All of these issues can easily be solved by allowing an option to use double-click for navigation.

It has been argued in the past that mobile devices and the web don't offer double-click for navigation and the introduction of mixed mode aims for more coherency but it must be kept in mind that on the web the user typically opens a link using the mouse, and at most copies or downloads it. In a file manager, selection, dragging, actions like copy/paste, rename, multi-select etc play a major role. Mobile devices, on the other hand, are a completely different scenario where everything is touch/tap+hold based and for selecting and copying items, the whole window adapts to display checkboxes etc. On a computer, the keyboard is used in combination with the mouse/touchpad and the usage of single-click/mixed-mode has thus far only hindered this combination.

Allowing an option to use double-click would not break the design of elementary, and will be incredibly helpful for those who need it.