Ask for confirmation when Closing Dolphin windows with a terminal panel running a program

Summary:
Ask for confirmation when Closing Dolphin windows with a terminal panel running a program.

FEATURE: 304816
FIXED-IN: 19.04.0

Test Plan:
# Open terminal panel
# Run `watch ls`
# Close Dolphin
# Observe confirmation
# Disable confirmation
# Repeat, observe no confirmation
# Enable confirmation in the settings
# Repeat, observe a confirmation

Reviewers: #dolphin, markg, elvisangelaccio, rominf

Reviewed By: #dolphin, elvisangelaccio

Subscribers: kfm-devel, elvisangelaccio, markg, ngraham, rkflx, broulik, #dolphin

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D10960