Introduce an icon macro

A macro for displaying an icon of the current or a chosen icon theme should be introduced. This avoids the need for using the Velocity macro display an icon that matches the icons in the rest of the UI. Parameters:

• Icon name
• Icon set (defaults to current icon set)
• Fallback to default (defaults to true)

Ideally, for the icon name an icon picker should be used but this won't be part of the first version.

A problem is that the font awesome icon macro already uses the obvious name "icon" so either that macro will be broken or this icon macro needs a new name.