Renames the subdevice property to just device, and has it
return the number of devices in the system in response to
a probe. This is useful both for using multiple capture
cards, and for detecting whether it's worth adding the
element to a pipeline.
Also cleans up the property descriptions.
_BAD_CFLAGS should always come first, then GST_PLUGINS_BASE_CFLAGS,
then GST_BASE_CFLAGS then GST_CFLAGS. Same for libs: first plugins
base libs, then GST_BASE_LIB then GST_LIBS.
