libdeng2|Animation: Animations can be paused and resumed
libappfw: PanelWidget waits until content is ready for drawing

A panel's opening animation is paused while the content is being
prepared for drawing. In practice, LabelWidget now declares its state
as an Asset, and PanelWidget observes when all visible labels are
ready.

Fixes multiple visual annoyances with dialog/popup layouts being
changed on the fly as the label content becomes ready.