generator: do not generate methods without (C)Name
generator,glib: added GPollFD and GSource types

GSource type was already there (but was not mapped by
the generator yet) so then the autogenerated methods
have been added manually inside the class after the
custom methods.

Other Source-related class are also generated and added
(but not mapped in the SymbolTable) to glib.
glib,generator: map MainContext type and expose members

This is needed to reference a MainContext from
external bindings, which need to create a
MainContext using a Handle
generator: include api files from XML
generator: new --gapidir flag to search for xml files
generator: fixed optional array parameters
generator: ignore private structs completely

Don't spam the log with these messages for private structs
(and don't count this in the statistics), as there are too
many. These kind of types are just empty structs marked as
hidden and private.