generator: Added constants to gapi

Unfortunately, gir marks all integers as gint regardless
of its size. We have to check if the value will really
fit into a int, that is why there is an automatic fallback
to long.
generator: fixed writeable and readable detection
generator: Added count param detection for return values
generator: fixed string array return type for virtual_methods
generator: Added long long conversion
generator: readable&writable attribs to be backwards compat