Add option to choose which view to close

Summary:
This Diff make configurable which view will close when toggling off
the split view mode, if it's the active one or the inactive one.

A new checkbox was added to the Dolphin configuration window,
and defaults to the original behavior.

FEATURE: 312834
FIXED-IN: 19.03.80

Test Plan:

Reviewers: ngraham, Dolphin, elvisangelaccio

Reviewed By: ngraham, Dolphin

Subscribers: elvisangelaccio, cfeck, kfm-devel

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D18040