Show recursive size of folders in statusbar

Currently the following is shown (example):

Selection: 5 folder, 3 files: 123,1 KiB (similar when nothing is selected)

So only the size of the files is shown, which can be confusing (see #756 (closed))

The idea whould be to as well show the recursive folder size (like it currently is done in the properties dialog)

E.g. like this:

Selection: 5 folder 10,0MiB, 3 files: 123,1 KiB

Or maybe better as a sum of both:

Selection: 5 folder and 3 files: 10,1MiB

Since it can take rather long to calculate a recursive size, it would be required to update the statusbar in fixed intervals, until the total size is known. (Like the properties dialog).

It as well should be possible to disable the "recursive size" feature. That might be beneficial on HDD drives or remote locations.