appDisplay: Separate PageIndicators to a separate file

appDisplay: Separate PageIndicators to a separate file

In order to cater for emoji panel usage, we want something like PageIndicators
except:
- It should have horizontal disposition
- It should not be animatable (?)
- It should not be reactive

Separated PageIndicators into a base, non-animated widget, and an
AnimatedPageIndicators that can be used on appDisplay.js. Reactiveness is
set through an extra method, and layout is set as a construct argument.