Miller Columns view for "File System Browser"

[slbtty]

Hi,

I have been trying to make the ColumnsView for dolphin, but do not succeed yet.
https://bugs.kde.org/show_bug.cgi?id=387455

In the same time, I noticed that I use kate to browser code extensively.

Then, an idea come to me: add an extra view mode for the "File System
Brower" → split the left panel into 2 and use the main editing area as
the 3rd panel of the miller columns.

Previously, I have been using ranger, which use miller columns as
default. When selecting a source file, it only needs 2 columns plus a
preview area. It looks pretty good.

Here is what it looks like:

https://imgur.com/a/TzRUP0f

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