This Lua Extension will get some meta data on the current movie
(or any movie) from the Internet Movie Database and shows it in
a simple dialog box.

Please consider this script more as an example than as a final
version of the IMDb extension.

You can now play with extensions, from the menu View > Extensions

These are the main UI files for extensions' dialogs.

Let's compile!

This contains:
- Extensions manager
- Extensions activation / deactivation functions
- Separate threads for each extension
- Lua specific functions for calling the script

New vlc.dialog library, that allows users to create scriptable dialogs.
The calling vlc_object_t should have register a callback for the variable
"dialog-event".

This patch defines a new type of interactions between the GUI and
so-called 'extensions'.
These extensions are dynamic plugins, that can be scripted, and they can
create GUI dialogs with widgets (like text fields, buttons, ...).

Most of them where already NULL, saying how meaningless it was.

 It still segfaults on broken streams (or on seeks #3153) and I am not
willing to spend more time on it.

The limit is arbitrary.

We have well over one thousand configuration items, not counting
dummy hint entries. So now, sort all of them once (well, twice),
and use binary instead of linear search for lookups.
config_Get*, config_Put*, var_Create(with INHERIT flag),
var_CreateGet* and var_Inherit* do such lookups.

While string configuration items have been protected for a while, float
and integer items weren't. This is now fixed.

There is still a small bug in some plugins setting ->b_dirty without
locking the configuration lock for writing.

Previously, we had one configuration mutex per module.
With a global read/write lock, resetting, loading, saving and
auto-saving the configuration becomes atomic (and use only one lock &
unlock pair). Also, multiple threads can now read the configuration
item of the same module at the same time.

Note that, as the earlier configuration mutex, only configuration item
values are protected. The list of items and their meta-data cannot
change while VLC runs (they're hard-coded in the plugin descriptors).