SocketSelector can't wait for writability

[mpartel]

Subject of the issue

Currently I can't write an efficient single-threaded server with SFML, because SocketSelector only lets me wait for a socket to become readable, not writable. If a socket's outgoing buffer gets full, I have to keep trying writes in a loop, which seems wasteful.

The solution is to make SocketSelector::wait() return when a previously full socket becomes writable again.

Your environment

• Checked code of SFML 2.5.1, 2.6.x and master - all pass only readfds to select().