[plug-in] Support custom icons in the TreeView

[vitaliy-guliy]

For now only icons from font-awesome are supported.
For that plugin provides class name as iconPath

{
    label: userName,
    iconPath: 'fa-user medium-yellow'
}

Tree View Item API describes that we have also to support icons from a file and pass it using resource URI.

This feature is a path of #2782

Demo plugin tree-view-sample-plugin should demonstrate the feature.

[azatsarynnyy]

should be fixed by #5057