Begin work on the PopupMenu entrance animation

Begin work on the PopupMenu entrance animation

This CL also refactors how animations work, particularly for the Drawer. I've
renamed DrawerAnimation to DrawerController and switched it from being an
Animation to having an Animation. I've also renamed Animation to AnimatedValue
to capture the idea that the class actually presents the value being animated.
Finally, I've factored AnimatedValueListener out of Drawer so that it can be
used by PopupMenuItem as well.

Finally, I've added a scheduleBuild convienence function to Component instead
of having to call setState(() {}), which has come up a couple times.

R=eseidel@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/c7d69836909bcd4c0b4beb4a2f71ac7164511104

[rafaelw, 2015-03-18 18:28:42]

rafaelw@chromium.org changed reviewers:
+ rafaelw@chromium.org

[rafaelw, 2015-03-18 18:28:42]

lgtm

[eseidel, 2015-03-18 18:30:24]

eseidel@chromium.org changed reviewers:
- rafaelw@chromium.org

[eseidel, 2015-03-18 18:30:24]

lgtm

[abarth-chromium, 2015-03-18 18:31:12]

Committed patchset #1 (id:1) manually as
c7d69836909bcd4c0b4beb4a2f71ac7164511104 (presubmit successful).