Improve notification layout

[boatbomber]

Changes:

• The width of the notification will be the minimum space required. (It used to do this, but was broken in Use FontFace and consistent text sizing #988.)
• The text is centered, making single line notifs feel less unbalanced.

Before & After:

• The text wrapping no longer shifts around as the notification animates in.

Before & After: (Slowed to show the effect more clearly)

before.mp4

after.mp4