Move internal events to a separate enum.

[xStrom]

As discussed in #826 this PR does the following:

• Moves all internal Event variants into a InternalEvent enum and adds the Event::Internal(InternalEvent) variant.
• Moves all internal LifeCycle variants into a InternalLifeCycle enum and adds the LifeCycle::Internal(InternalLifeCycle) variant.

Closes #826

[xStrom]

This was a Scroll bug, but it's not easy to fix. More info in #834.

[luleyleo]

Looks good to me

[cmyr]

broadly good, but I'm unsure about Size. I haven't read #834 yet, which may be relevant? But my inclination is to not make size private, but maybe change its behaviour.

[cmyr]

There's an ongoing question about whether or not this should actually be more widely used, and sent down the tree by default; and even if not I do think of it as public API, custom widgets would regularly want to handle it I think?

[xStrom]

Well Size is only sent to the root widget and it's window resize, not layout size changes. By changing its behavior, you mean that every widget should be informed about window resizing? In that case, it should at least be renamed to WindowSize. Or you're thinking about changing its meaning?

[cmyr]

yes, it could be WindowSize. Basically I think that this is an event that applications might want to handle explicitly, such as by recentering the contents of a scroll view.

[xStrom]

Okay, what about propagation? Still send it only to the root or to everyone?

[luleyleo]

Wouldn't it be more useful if widgets could get an event when their own size changes?
Not sure what the window size would really be useful for.

[xStrom]

I'm somewhat skeptical of the value of the window size event as well. However for now I brought it back to the public Event enum under the better describing WindowSize name. We can discuss its value further, but I don't want that holding up this merge.