Fix list scrolling

[mat007]

Hey!

This pull request fixes the list scrolling.

Applying this this patch and running go run _examples/widget_demos/list demonstrates part of the issue.
The following video shows what happens when manually scrolling to the bottom, then clicking the button to select entry 20: the scrolling to show the entry ends up with it well inside the view instead of the first element at the top of the view.

Changing this line in _examples/widget_demos/list/main.go

	numEntries := 40

to

	numEntries := 300

then running the example again, and clicking the button to select entry 20, highlights another problem that cannot be captured very well in a video: the scrolling moves up and down every frame resulting in a constant blurred list view.

The fix in this pull request starts by renaming the scrollcontainer.ContentRect method to scrollcontainer.ViewRect because it’s actually what it is: the rect for the current visible part of the container, not of the underlying part with all the content.
It then introduces a «real» scrollcontainer.ContentRect method, that’s needed to properly compute the scrolling when making a widget visible in the view.

The first 2 commits could probably be merged into the 4th, I can do that if needed.
Or even squash all the commits…