Up to now the SDL audio driver attempted to use the default recording
device. This might not be what users want or expect, especially since the
actually used device is not visible anywhere. So if recording does not
work for the users they have no way to find out what's wrong.

Extend the settings screen of the SDL driver with a combo box that allows
to select the input device to be used. Store the selected device name in
a new attribute called "inputdevice" in the "audiosdl" section of the
configuration file.

Use the information from the configuration when attempting to inialize
the input device. Fall back to the default device if that does not work.

(cherry picked from commit 33139b9)

Provide the setting "[System Default]" which instructs the SDL driver to
use the default device of the system as the input device. In the
configuration file this option is represented as an empty string. This
should play well with the current existing configuration of the users.

(cherry picked from commit 29c43c2)

Let users configure the output device that's used by the SDL driver.
Code-wise the implementation is very similar to the input device
configuration.

Use a `QComboBox` instead of a `QLineEdit` for `m_device` and rename it
to `m_playbackDeviceComboBox`.

Rename `s_defaultInputDevice` to `s_systemDefaultDevice` because it is
used in the context of playback and input devices.

(cherry picked from commit 1ab45e4)

Make sure that labels are always shown by setting the row wrap policy of
the form layout to wrap long rows.

(cherry picked from commit a123d0e)

Rename "Device" to "Playback device" to make clear what the setting
refers to.

(cherry picked from commit 1f0cda4)

Introduce const expressions to get rid of repeated strings with a risk
of typos.

(cherry picked from commit f9ea970)

Apply some more changes that have been made to `AudioSdl` in the
recording branch.

Also use a conditional ternary operator for the input device setup.

Move the population of the input and playback device combo boxes into
the methods `populatePlaybackDeviceComboBox` and
`populateInputDeviceComboBox`.

Sort the devices names alphabetically in the input and playback combo
boxes. The default devices is always shown as the first entry.

Use `AudioDeviceSetupWidget` instead of `QObject` to translate "[System
Default]".

Fix copy/paste error in comment.