Adds description management in Style and Rule

It is now possible to manage the Description of Styles and Rules in the simple style editor. The Description contains titles and abstracts that are associated to locales. For one locale, you can have zero or one title and zero or one abstract.
What is added here is mainly the UI mechanisms. Description was already implemented and working in orbisgis-core - I've only made minor changes in this library.
The UIs are created in a dedicated class that manages a Description instance directly. As it is not linked to a Rule or a Style, I've been able to use it in both PnlStyle and PnlRule. It will be possible to use them for symbolizers too and to include them freely in the advanced style editor thanks to that.

By default, the UI behaves this way:
- If there is not any abstract or title, the default locale is selected.
- If there is one or more abstracts or title and one of them is associated to the default locale, the default locale is selected.
- If there is one or more abstracts or title but none of the m is associated to the default locale, the first found one is selected.

If a Locale has a title or abstract and you want to remove it, you just have to remove the text in the field. There won't be an empty string in the resulting style.

This pull request is related to #486.

The diff looks huge, but I've edited the .po file...