Bring language support infrastructure back to KDevelop4.

The new language support framework is modelled after project management architecture we have already with following components:
- LanguageController to load languages when code files are activated
- Language - the class to represent particular language (in the future with common stuff like background parser, etc.)
- ILanguageSupport - the extension interface to be implemented by actual language support parts
The main features of this framework:
- as many language supports as necessary may be loaded at the same time
- language supports are loaded on demand (depending on the mimetype of current file)
- several language supports can be activated for the current file (useful for mixed-source stuff like html+php, html+ruby, c+sql, doxygen+c++, etc.)