Make the settings dialog work in the frameworks branch

Make the settings dialog work in the frameworks branch

The KF5 version of KPageDialog has no virtual slot
"slotButtonClicked(int button)". Its kdelibs 4.x counterpart had such
a slot, which was connected automatically to the corresponding signal.

This slot was overriden by

DolphinSettingsDialog::slotButtonClicked(int button)

which was responsible for applying the changed setting and restoring
the default values if the corresponding button was clicked.

The lack of the buttonClicked(int) signal and the corresponding slot
caused the problem that clicking a button in the settings dialog had no
effect.

This patch makes the functions applySettings() and restoreDefaults()
functions slots, and connects them directly to the "clicked" signal of
the corresponding buttons.

BUG: 335709
REVIEW: 118576