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:

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

Observed:
  # Spectacle runs in background and the screenshot gets saved to clipboard
  # Spectacle never quits

Reviewers: davidre

Subscribers: ngraham, #spectacle

Tags: #spectacle

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