Seeks in the stream by the given
offset, modified by
Attempting to seek past the end of the stream will have different results depending on if the stream is fixed-sized or resizable. If the stream is resizable then seeking past the end and then writing will result in zeros filling the empty space. Seeking past the end of a resizable stream and reading will result in EOF. Seeking past the end of a fixed-sized stream will fail.
Any operation that would result in a negative offset will fail.
cancellable is not null, then the operation can be cancelled by triggering the cancellable
object from another thread. If the operation was cancelled, the error g_io_error_cancelled will be returned.
optional Cancellable object, null to ignore.
true if successful. If an error has occurred, this function will return false and set throws appropriately if present.