Document custom source properties for external developers

[virn]

We'll need to explain how developers can use the enablehtmlprop:2 and the custom xsplitprops.html when debugging their custom plugins.

We would need to add this on the tutorials section on the website.

[virn]

I've also consulted with @SML-MeSo about the possible exposure that enablehtmlprop may bring and here are the other methods.

• enablehtmlprop:-1 don't use html props but subscribe to OnShowSourceSettings
• enablehtmlprop:0 don't use html props and don't subscribe to OnShowSourceSettings
• enablehtmlprop:1 set html props content and subscribe to OnShowSourceSettings
• enablehtmlprop:2 if xsplitprops.html file exists use it, else extract html props content to temp file and set file url and also subscribe to OnShowSourceSettings
• enablehtmlprop:3 same as 2 but does not subscribe to OnShowSourceSettings

As discussed, it is safe for us to use the _enablehtmlprop _ parameter.

Additionaly, once the customized HTML is added to the CDN, I will be creating a link for this on the website.