Closes #5487

This PR adds support for setting the palette.theme setting via a theme plugin. It also extends support for this setting to provide custom icons to nodes.

The docs need updating to provide full details, but here are my working notes.

editorTheme.palette.theme is a way for custom colors to be specific for individual nodes identified by type or category (using a regex to match multiple where needed).

    palette: {
        theme: [ // Override node colours - rules test against category/type by RegExp.
            { category: ".*", type: ".*", color: "#f0f" }
        ]
    },

• the rules are evaluated in order - the first matching rule applies. This allows for a catch-all rule to be at the end of the list
• each rule can now include an icon property to set the icon of any matching nodes - see notes below on how icons are provided.
• when looking for an override for a node's icon, it will ignore rules that don't include an 'icon' property - and the same is true for 'color'. This means you can separate out color/icon rules, or combine them - lots of flexibility.

Theme Plugins

A theme plugin can now include this setting, meaning the settings.js file does not need to be touched. It also means the plugin can bundle any icons it wants to apply in its own package - making distribution much easier.

The plugin .js file (that registers itself) should contain the palette setting:

module.exports = function (RED) {

    RED.plugins.registerPlugin('test-theme-plugin', {
        type: 'node-red-theme',
        // scripts: [],
        // css: [ theme.css' ],
        palette: {
            theme: [
                { type: 'inject', color: '#e4d725' },
                { type: 'inject', icon: 'banana.png' },
                { type: '.*', color: 'red' }
            ]
        }
    })
}

In the same directory as the plugins .js file should be an icons directory. This should contain (with no further sub-directories iirc) the icons the plugin provides.

├── icons
│   └── banana.png
├── package.json
└── theme.js

The icon can then be referenced by name without including the icons part of the path.

To enable a theme plugin, edit settings.js and set editorTheme.theme to the name of the theme - test-theme-plugin in the example above (the first arg passed to the registerPlugin call).

Useful debug tip: if icons aren't loading as expected, in the browser console, run RED.nodes.getIconSets(); - this will return an object listing all of the custom icons known to the editor, organised by which module is providing them. Check the theme plugin shows up and lists the icons you think it should.