New c++ library

@foolab , I don't think there is any problem with that. These are headers to be included by other projects so it makes sense to use system includes. When compiling the own code locally the -I flags to the system installation are not passed so it should be safe. Also, I've checked with existent Qt headers and it seems they also use the system includes.

@deztructor , I'm not sure I'm understanding your comment. The classes are already prefixed with Grilo and the library is called grilo-qt. It makes sense that the global include is called as such. "grilo" is the name of the original C library. Following your suggestion, the include should be, in any case, <GriloQt/global.h> or something similar and the developer would be forced to add all the includes, then, with the GriloQt directory. I would leave it as it is now.