lavfi: add const to AVFilterContext.filter.

lavfi should never modify the filter through that pointer.
lavfi: add const to the pads parameter of avfilter_pad_get_name/type
lavfi: merge avfiltergraph.h into avfilter.h

We do not support using filters without AVFilterGraph in practice
anyway, so there is no point in pretending we do.
lavfi: replace avfilter_open() with avfilter_graph_alloc_filter().

Since we do not support "standalone" filters not attached to an
AVFilterGraph, we should not have a public function to create such
filters. In addition that function is horribly named, the action it does
cannot be possibly described as "opening" a filter.
lavfi: deprecate avfilter_graph_add_filter().

Since this function adds a standalone filter to a filter graph and we do
not support creating such filters, there is no reason for this function
to exist.
lavfi: add AVFilterContext.graph.

It will be useful in the following commits.
lavfi: make avfilter_free() remove the filter from its graph.
lavfi: add avfilter_init_str() to replace avfilter_init_filter().

Drop the unused opaque parameter from its signature.
lavfi: add avfilter_init_dict() for initializing a filter with a dict.
lavfi: mark filters with dynamic number of inputs or outputs with special flags

This will be useful in avtools in the following commits.
Any other caller might also want to know this information.
lavfi: add a function for counting elements in AVFilterPad arrays.

The caller needs to know what valid indices can be passed to
avfilter_pad_get_name/type.
lavfi: change the filter registering system to match the other libraries

Removes an arbitrary hardcoded limit on the number of filters.
lavfi: do not segfault on NULL passed to avfilter_get_by_name()
avconv: update to the new options syntax.

The separators were changed from ',' to '|' in all the filters.
avconv: print filter options in -h full output.
avtools: move media_type_string() from avprobe to cmdutils.

It will be useful outside of avprobe.
cmdutils: allow -h filter=<name> to print information about a filter.