Caps and Options update

[Guillaume Emont <gemont igalia com>]

Following the discussions we've had, I've advanced the development of
GrlCaps and GrlOperationOptions.
I've pushed the current state of things to my gitorious repository [1]
The stuff is in src/grl-caps.{h,c} and src/grl-operation-options.{h,c}.
I have also added some helper tools to handle GValues in
grl-value-helper.{h,c}

I'd be happy to have some feedback on that before I start integrating it
into grl-metadata-source.c.

Some notes on it though:
 - I have put caps and pagination in the caps, even though they might
not be needed here (because we're likely to want these to be mandatory).
This might change in the future, and is like that mainly to illustrate
how caps and options interact.
 - the _set_value() methods are inconsistent in the way they take
ownership of their parameters. I plan to change that, probably to always
copy @value, since taking ownership of it is assuming it was created
using grl_g_value_new(), which might not always be true.
 - for the options and caps that are defined, I think we should have
default values. I'm still hesitating on whether to put that in
initialisation or in getters.

Cheers,

Guij

[1]
https://www.gitorious.org/~guijemont/grilo/guijemont-sandbox/trees/caps_and_options