[grilo-qt] New library to allow easy use in C++

Reordered the code to allow splitting in a new C++
library.

Now, the QML library depends on it.

Also, created 2 new RPM packages to hold the
library and the needed development resources and
renamed the QML plugin package.
[example] New very basic C++ example
[devel] Headers installation moved to GriloQt and corrected .pc file
[example] Completed a little bit more the simple example
[example] Removed left qWarning
[rpm] Enhanced description of the main package
[registry] Removed unneeded declarative interface from GriloRegistry
[qtgrilo] Remove declarative bits from the C++ library

- Implement a new GriloModel wrapper called DeclarativeGriloModel
- Removed GriloModel::get() and moved to its declarative counterpart.
- convert qmlInfo() to qWarning() in GriloMedia
Merge pull request #16 from Igalia/new-c++-library

New c++ library