add: animated pop-up text.
add: statistic changes are shown using pop-texts.
mod: Main-menu buttons use Config rather than hard-coded strings.
add: 'extend' helper function to deal with property copying.
mod: derive uses extend.
mod: 'on'-helper will return the given object unless the argument functions returns something.
fix: PopText#next() uses given style.
mod: small pop-text animation refinements.
ren: PopText.next -> chain.
add: PopText documentation.
add: highlighted statistic changes.