Use PIMPL for GUI Events in Fortress

[scpeters]

Desired behavior

I noticed some fields that would be nice to add to the *ClickToScene events in #209 (comment), but this breaks ABI since we aren't using the PIMPL pattern.

• https://github.com/ignitionrobotics/ign-gui/blob/ign-gui5/include/ignition/gui/GuiEvents.hh

So, we should use PIMPL in this header and move the implementations to a src/GuiEvents.cc file.

Alternatives considered

Keep it as is and only add new fields in unreleased code. This is less flexible.

Implementation suggestion

See src/Application.cc for an example.

Additional context

https://en.cppreference.com/w/cpp/language/pimpl

[scpeters]

done in #253