Add some dartdoc to basic widgets

Got the check the wrong way in the last patch.

TBR @abarth

Ignore hover events.

These docs are largely duplicated from the cooresponding rendering classes. I'm
not sure how we can avoid repeating ourselves.

Fixes issue with physics debug drawing in sprites

Expose markNeedsBuild on MimicOverlayEntry

We expose this function on OverlayEntry to let you drive animations in the
overlay. The same use case applies to MimicOverlayEntry.

Export the new pointer event classes.

Parent data fixes

Add some missing functionality to ParentData subclasses.

Fixes #474

Update Flutter API docs continuously

The patch teaches Travis how to update our API docs continuously as we land
patches in master.

Repeated background images should be clipped to the container

Fixes #766

Add a completer to MaterialPageRoute

LayoutId should default to using its id as its key

This default seems likely to be benefitial because typically the LayoutIds
coorespond to semantic slots in a bespoke layout.

Popup menu splash doesn't go to edge

Fixes #608

Now the splash goes to the edge. Also, fold popup_menu_item.dart into
popup_menu.dart for simplicity.

Fixes #266

Implement ImageRepeat

Cleanup for the Pointer changes.

Fixes #158

PointerInput refactor

Instead of PointerInputEvent having a "type" field, we now have a
different class for each pointer type.

This has ripple effects throughout the system.

I also did code cleanup in affected files while I was there.

Remove PaintingCanvas

Remove outdated radial_reaction.dart

We can just use Canvas now.  The two are the same thing.

We now draw radial reactions using Material.

Replace AnimationTiming with CurvedPerformance

MeanPerformance

Also, replace places that use the term "create performance" with
"create performance controller".

Refactor PerformanceView listeners.

Primarily, this rearranges how listeners are handled internally by the
various PerformanceView classes so that there's not so much redundancy.

It also fixes ReversePerformance and ProxyPerformance to not leak.
Previously, they never unhooked their listeners, so they'd leak until
the entire chain of performances was killed. Now, they unhook as soon as
they themselves have no listeners left, which is more idiomatic.