Fix missing + in isIdString()
Fix width assignment in Inputs and Slide
Add setWidthsToArray() to Slide
Update equals() to compare widths array

* fix/update spec to use equals()
Update defaults in main.js
Show widths inputs only if editing them
Lower min integerValue in updateIfNeeded()
Call preview() only if slide has changed
Fix Template's setInput() using prop()
Create Limiter class, limit widths in UI