on_event: expand certain event parameters

[ivan-mogilko]

#2468 task required to add x,y coordinate parameters for on_mouse_click.
But there are also eEventGUIMouseDown and eEventGUIMouseUp events handled by on_event that also may require these.
For instance, some of the game templates are handling these, and use mouse.x/mouse.y to know the position, although, since on_event is scheduled, this position may already be different.

Another thing, there's event eEventGameSaved, but there's no way to tell if it was successful or not. Right now the engine simply displays a standard hardcoded message in case a save file failed to open. I think it should not be doing that, and let game developer handle this instead. This may be done by passing an extra parameter along eEventGameSaved, which tells the boolean result of the save operation.