Handle resource requests through singleton.

Many applications have multiple objects dealing with audio, and each
needs to request audio resources from resource policy manager. Resource
policy manager handles audio resources per-manager. Each ResourceSet
instance creates new manager, which causes applications having multiple
audio objects to fight for the audio resources internally, even though
the streams are played from the same process id.

To overcome this in QtMultimedia applications, handle all ResourceSet
operations through singleton. This way one client has only one manager
id registered, and resource acquiring and releasing can be limited to
only when no resources are acquired or all resources are freed. To
reduce unnecessary noise from resource policy plugin to the clients,
keep track of client states and only notify clients which themselves
request for resources etc.
Implement more full resource policy features.

 * Handle released by manager properly.

Signal resourcesReleasedByManager is different from signal
handleResourcesLost and needs to be handled separately.
Signal handleResourcesLost means some other client has acquired the
resources, thus the resources are lost from us, but if the other client
frees the resources, we will automatically get the resources back.
With resourcesReleasedByManager we lose the resources, and resource
manager releases them as well (same as if client would call release()),
so only way to re-acquire resources in latter case is calling acquire()
again.

This distinction is useful for example in cases where user is listening
to music with headphones connected and removes the headphones, then the
music player shouldn't continue playback until user requests so. But if
user is listening to music when phone call is received, it is convenient
to resume the playback automatically after the call.

 * Implement availability changed.

Availability changed is information whether resources the client is
interested in are available (no higher priority resource classes have
acquired the resources).

 * Implement missing resources released.

Emit resourcesReleased signal when receiving corresponding signal from
libresource-qt.

 * Add mechanism to change used resource class.

Check for environment for special variable, and if the variable has
proper data, use that as the resource class. Can be set with for example
qputenv("NEMO_RESOURCE_CLASS_OVERRIDE", "game");
Resource policy for QAudioOutput.
Only call pa_cvolme set on stream creation if volume explicitly set.

Streams without a custom volume set use suitable volume selected
by the PulseAudio server. This is usually the correct choice, since then
stream-restore module can apply old volume to the stream. Also with
Sailfish explicitly setting the volume to streams breaks system volume.
Resource policy support for QSoundEffect.

Since sound effects are something short and mixed with other audio, do
not acquire resources explicitly. Follow the resources availability
information to determine when it is ok to play the sound effects.

When client has registered itself to resource manager, client's streams
are classified properly. If no higher priority client has acquired the
resources, isAvailable() is true, and sound effects can be played. We
do not explicitly acquire the resources, since then other clients with
the same resource class would lose the resources, thus not possible to
have second client play music with QMediaPlayer class while our client
would just want to play simple sound effects.
Set volume only if volume was explicitly set.
Add resource release timer when pausing or stopping media.

If media is paused, stopped, or end of media, if state hasn't changed to
playing state, release the resources. The idea with the timer is that if
for example media player is playing one song which ends, there won't be
release()/acquire() unnecessarily if next song is immediately set as
media and started playing.
Enable NEMO_AUDIO