tools/gstreamer-completion: Updated to work with the binary registry

The original registry was in xml format (~/.gstreamer-*/registry.xml).
A binary registry format was added in 2007 (commit ebf0c9d3) and made
the default in 2008 (commit 3f39fd7e). It looks like you can still
choose at "configure" time to use the xml registry instead.

This change to gstreamer-completion should work with either format
because it parses the output of "gst-inspect" instead of reading the
registry file directly.

Note that _gst_launch no longer needs an explicit "return 0" because,
unlike the previous grep command, compgen always returns 0 (unless a
genuine error occurs).

Just like the previous implementation by David Schleef, this "only
completes names of features, but that's 90% of what I want it for."
tools/gstreamer-completion: Support gst-inspect, and gst-launch element properties

Completes options like "--gst-debug-level" and the values of some of
those options; completes gst-launch pipeline element names, property
names, and even property values (for enum or boolean properties only).

Doesn't complete all caps specifications, nor element names specified
earlier in the pipeline with "name=...".

The GStreamer version number is hard-coded into the completion script:
This patch is off the master branch and has the version hard-coded as
"1.0"; it needs to be updated if backported to the 0.10 branch. You
could always create a "gstreamer-completion.in" that has the appropriate
version inserted by "configure", but I'd rather not do that. The
hard-coded version is consistent with the previous implementation of
gstreamer-completion, which had the registry path hard-coded as
~/.gstreamer-1.0/registry.xml.

Note that GStreamer 0.10 installs "gst-inspect" and "gst-inspect-0.10".
"gst-inspect --help" only prints 4 flags (--help, --print, --gst-mm,
gst-list-mm) whereas "gst-inspect-0.10 --help-all" prints the full list
of flags. The same applies to "gst-launch" and "gst-launch-0.10".
GStreamer 1.0 only installs "gst-inspect-1.0", not "gst-inspect".

Requires bash 4; only tested with bash 4.2. Requires "bash-completion"
(which you install with your system's package manager).

Put this in /etc/bash_completion.d/ or in `pkg-config
--variable=compatdir bash-completion`, where it will be loaded at the
beginning of every new terminal session;
or in `pgk-config --variable=completionsdir bash-completion`, renamed to
match the name of the command it completes (e.g. "gst-launch-1.0", with
an additional symlink named "gst-inspect-1.0"), where it will be
autoloaded when needed.

test-gstreamer-completion.sh is (for now) in tests/misc -- is might be
worth creating "tests/check/tools", with all the necessary automake
boilerplate, and moving test-gstreamer-completion.sh there, and have it
run automatically with "make check".

IF YOU'RE NEW TO BASH COMPLETION SCRIPTS
----------------------------------------

"complete -F _gst_launch gst-launch gst-launch-1.0" means that bash
will run the function "_gst_launch" to generate possible completions
for the commands "gst-launch" and "gst-launch-1.0".

"_gst-launch" must return the possible completions in the array variable
COMPREPLY. (Note on bash syntax: "V=(a b c)" assigns three elements to
the array "V").

"compgen" prints a list of possible completions to standard output. Try
it:

    compgen -W "abc1 abc2 def" -- "a"
    compgen -f -- "/"

The last argument is the word currently being completed; compgen uses it
to filter out the non-matching completions. We put "--" first, in case
the word currently being completed starts with "-" or "--", so that it
isn't treated as a flag to compgen.

For the documentation of COMP_WORDS, COMP_CWORD, etc see
http://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html#index-COMP_005fCWORD-180

See also:
* http://www.gnu.org/software/bash/manual/html_node/Programmable-Completion.html
* http://www.gnu.org/software/bash/manual/html_node/Programmable-Completion-Builtins.html

The bash-completion package provides the helper function
"_init_completion" which populates variables "cur", "prev", and "words".
See
http://anonscm.debian.org/gitweb/?p=bash-completion/bash-completion.git;a=blob;f=bash_completion;h=870811b4;hb=HEAD#l634

Note that by default, bash appends a space to the completed word. When
the completion is "property=" we don't want a trailing space; calling
"compopt -o nospace" modifies the currently-executing completion
accordingly. See
http://www.gnu.org/software/bash/manual/html_node/Programmable-Completion-Builtins.html#index-compopt