Embedded samples on-demand loading

[damyanpetev]

The frame loads the entire samples app and topics with multiple samples a bit to load.
It'd be a performance improvement (especially in dev mode) if the embedded sample can load on-demand as the page is scrolled through.
Not too noticeable on production and there aren't that many topics with a lot of samples, just something that can be looked into in the future.

[tiliev]

Please follow these steps for implementing lazy loading in a topic:

1. Generally the first sample should not be lazily loaded. Add loading class to the sample container and onload="onSampleIframeContentLoaded(this); on the iframe.
2. On all of the sample you'd like to load lazily, add

• loading class to the sample container
• lazyload class to the iframe
• rename the src of the iframe to data-src
• you shouldn't have onload="onSampleIframeContentLoaded(this);"

P.S. Consider automating this, e.g. using C# and Html Agility pack.

[zdrawku]

@Aleksandyr and @HristoP96 think of a way to automate this process for the current topic.
I am going to add a readme section about the lazy loading configuration for all upcoming topics

[HristoP96]

All the PRs related to this issue are merged.