FIX: Close spectacle if it's started with: -b -c

Summary:
Spectacle should close if both -b and -c are set

Preparation to fix this one:
CCBUG: 411263

Test Plan:
start spectacle:
spectacle -b -c

Expected:

1. Spectacle runs in background and the screenshot gets saved to clipboard
2. Spectacle closes itself after the notification disappears (or has been closed by the user)

Observed:

1. Spectacle runs in background and the screenshot gets saved to clipboard
2. Spectacle never quits

Reviewers: davidre

Subscribers: ngraham, Spectacle

Tags: Spectacle

Differential Revision: https://phabricator.kde.org/D23522