Update qt/optionsdialog.cpp with Qt5 connect syntax

Summary:
Updates to use Qt5 connect syntax over Qt4.
See https://wiki.qt.io/New_Signal_Slot_Syntax for details.

Partial backport of Core PR13529
https://github.com/bitcoin/bitcoin/pull/13529/commits/f78558f1e39198779bdb17e2b0e256fb99ad4b28#diff-d3299add50f0fbf37c2edf0f9ce2ac13

Test Plan:

make check
./bitcoin-qt
Settings -> Options

Select the Prune option. Verify that there is a warning at the bottom of the options window alerting the user that a reset is necessary to changes to take effect in red text.
Click the Reset Options button. Verify that a restart warning window pops up with yes and no options. Verify that selecting no closes the pop up window and selecting yes shuts the gui down and resets options to the default setting.

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4348