Implemented an SFML-level event filter for Linux.
Added missing documentation.
Don't try to generate a wheel event if generating a mouse button event failed because we don't want those.
More handy aliases.
Optimise the event filtering with the help of Xorg's own event filter.
Optimise event processing when we are not interrested in joystick events.
Use a different method to process OS events in blocking mode
rather than using a boolean flag to chose between the two ways of processing.
Fixed a bug due to Xorg not generating ButtonRelease events when no corresponding ButtonPress was generated.
That, and also a typo.
We not being interrested in an event does not mean the parent windows are.

This should fix the out-of-nowhere performance drop that happens when
an event is filtered on Xorg level's.
Fail early if JoystickButton or JoystickAxis are filtered.
Clarify documentation.