Add isFromClient to SelectionEvent

[Legioth]

When listening to selection changes from e.g. Grid, it's sometimes necessary to know whether the event was triggered programmatically from the server or originated from the client.

One specific use case for this is when HasUrlParameter.setParameter selects the corresponding item in a Grid, and a selection event from Grid causes navigation the the URL corresponding to the selected item.

isFromClient is currently defined in ComponentEvent, so SelectionEvent should either be changed to inherit from that class, or alternatively isFromClient could be extracted to a mixin interface or an even more abstract base class.

[pleku]

SelectionEvent is also missing the source, could be trivial to fix together: #3404

[denis-anisimov]

Fixed in #3718