Checks if this can be read.
Note that some stream types may not be able to implement this 100% reliably, and it is possible that a call to read after this returns true would still block. To guarantee non-blocking behavior, you should always use read_nonblocking, which will return a g_io_error_would_block error rather than blocking.
true if this is readable, false if not. If an error has occurred on this, this will result in is_readable returning true, and the next attempt to read will return the error.