Use lazy loading for content in tabs and sections

[bekand]

Terms

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[bekand]

The ifThenElse function increased readability in some cases, but every time it was used, both of it's arguments were evaluated, because of Elm's functional nature, and strict evaluation, so I removed the function and I'm using a simple if..then..else instead.
I also rewrote the Tabs and Section components, so the function producing their inner content doesn't get evaluated unless it's actually shown.
These two changes increase the performance of the UI significantly, especially for large models and/or long expressions.