Improve custom icon support

[amartyn-everlaw]

Buttons already accept custom icons, but most other components do not. This changes allows all components that have icon props to use custom icons, similar to Buttons.

The main changes are

• Adding a generic icon type in typescript to avoid repetitive code
• Adding an IconUtils class that has a method getJSXIcon. This method is based on renderIcon in the Button class. For strings, it will create a span with the string as the className. For React.ReactNode objects or functions that return a React.ReactNode object, it will wrap the object in a span and return it
• Updating components to use getJSXIcon instead of creating span or i icons directly

I don't know if this will be merged, but I thought it might useful to other people who want to use custom icons

[amartyn-everlaw]

@mertsincan Any thoughts on this?

[mertsincan]

Hi @amymartyn,

Nice PR! Can you update the documentation and codes according to conflicts?

Best Regards,

[amartyn-everlaw]

@mertsincan OK, conflicts have been fixed

[mertsincan]

Thanks a lot for your effort, @amymartyn ;) Good idea, Nice PR ;)