Implement Notification animation with CSS (#13660)

Default opacity (90%), fade duration (400ms) and fade delay (1000ms) are
defined in the base theme.

“In” animations can be defined by using a dependent stylename
‘animate-in’ (prefixed with the widgets own primary stylename), e.g.
'v-Notification-animate-in'. That CSS class should then have a CSS
animation specified, and the CSS animation name should contain
'animate-in', e.g.

.v-Notification-animate-in {
animation: notification-animate-in 400ms;
}

@keyframes notification-animate-in {
...
}

“Out” animations are defined similarly, but with 'animate-out' as the
keyword.

The base theme contains mixins for declaring keyframes rules as well as
animation values, which include needed vendor prefixes.

Refactored deprecated GWT API’s to use newer API’s.

Change-Id: Iba929e8de8b6ce100852d472d65c9c9d0630c48f
Fix copyright header in AnimationUtil

Change-Id: I768d93e8eb7f4b6415f17c80bc4564053313dc56
Use Sass Compiler version 0.9.6

Change-Id: I92bd1a76c79785ee5d2cdc7000cb4746ef25192e
Merge branch 'valo' into 7.3

Change-Id: I0c1e278e153dc1ca4de840a5cec6ca83cb126332