Scott Harvey authored Mar 18, 2018 and rk flx committed Mar 18, 2018

Summary:
Added "break;" and "return;" where needed to resolve fallthrough warnings

- Resolve compiler fallthrough warning in Main.cpp, line 135
Added call to `quitOnLastWindowClosed(false)` at the end of `BackgroundMode` block,
ensuring this flag is set directly, rather than allowing it to be set via fallthrough into the
`DBusMode` block beneath. Added `break` to properly end the `BackgroundMode` block.

- Resolve compiler fallthrough warning in SpectacleCore.cpp, line 212
Added explicit call to emit 'allDone()' signal, rather than allowing execution to fall through
into block for DBusMode, which previously emitted the signal. This change also removes the
emission of signal 'grabFailed()' during a BackgroundMode failure, which only happened due
to the fallthrough into the DBus block, but signaling on the bus is not needed in `BackgroundMode`.

Test Plan:
  - Compile Spectacle
  - Ensure warnings at Main.cpp:135 and SpectacleCore.cpp:212 are eliminated

Reviewers: #spectacle, rkflx, ngraham

Reviewed By: #spectacle, rkflx, ngraham

Differential Revision: https://phabricator.kde.org/D10936