Added functionality to enumerate capture devices on all platforms

If the environment variable OWR_USE_TEST_SOURCES is set the list of sources will include test sources.
local: Add copyright header to new files
local: Don't list monitor sources from PulseAudio
tests: Add manual device selection to test-self-view

Off by default since it's probably not needed other than for device
listing testing.
local: Schedule device listing in Owr mainloop context

This moves the device discovery step into a mainloop, rather than only
scheduling the callback.

Also the callback was scheduled using g_idle_add() that would use the
default mainloop context, which is now changed to use the Owr main
loop/context.
local: Move device index to exist only on OwrLocalMediaSource

This is not a property that makes sense on the more generic
OwrMediaSource.
local: Make Linux audio device selection actually work
local: Pick Dalvik over ART if available

Both might be available on pre-Lollipop Android devices, and the default
in those cases is Dalvik. This does mean we won't support people who've
forced ART on older devices, but such is life.
local: Fix up build with device listing

This makes Linux and Android builds work. Needs to be tested and tweaked
for OS X.
local: Namespace the _get_avf_*() functions

Adding a _owr should avoid accidental namespace collisions in case of
static builds.
owr: Add transfer annotation for new internal API
local: Use g_warning instead of g_error

g_error() will kill the process
local: Use Owr main context for PulseAudio glib mainloop integration
local: Don't list v4l2 devices that we are unable to open
local: Use g_list_prepend() instead of g_list_append()

g_list_prepend() is O(1), whereas g_list_append() is O(n).
local: Explicitly specify void argument lists for functions
local: Fix up some Android exception handling
build: Device listing on Apple needs to link to AVFoundation
local: Use CoreAudio for audio capture device listing

While listing AVFoundation devices is simpler, there doesn't seem to be
to get the underlying CoreAudio device ID from the AVFoundation device,
which is what we need while instantiating an osxaudiosrc.
local: Autorelease device list for AVFoundation devices