Add an option to show tabs from last time when Dolphin starts

Summary:
All modern web browsers offer a function to show tabs from last time when a browser starts, and many apps today
restore their prior state when they're launched. This patch implements thatfunctionality as an option and turns it on by default.

The settings window is accordingly adjusted to be clear about what applies when:

FEATURE: 413564
FIXED-IN: 20.08.0

Depends on D25106
Depends on D25219

Test Plan:
With the new setting turned off:

• No behavioral changes at all

With the new setting turned on:

• When launched from the GUI or CLI without any URLs, dolphin restores session
• When rebooting with Dolphin open, it restores session normally after the system comes back (i.e. no behavioral change here)
• When launched with URLs, Dolphin window is opened showing those URLs instead of restoring session
• When Dolphin is already running and a new window is opened, that new window shows a single tab with the same URL as was visible in the previously-open Dolphin instance (i.e. no behavioral change here)
• "Open Containing folder" functionality in other apps works regardless of whether or not Dolphin is running

Reviewers: Dolphin, VDG, feverfew, meven, elvisangelaccio, ndavis

Reviewed By: Dolphin, VDG, feverfew, elvisangelaccio, ndavis

Subscribers: davidedmundson, ndavis, intika, feverfew, kfm-devel, ngraham, broulik, Dolphin

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D11382