Support icon packs from different sources

[rsubtil]

The plugin uses Xelu's Icon Prompts since the beginning. However, a decision was made to ship only the dark variant of such assets, to avoid shipping plenty of likely unnecessary assets by default.

But this created the problem of not offering the choice to the user of using the light variant instead. And, as time moved on, Kenney designed a new set of icons which are also of high quality and good candidates to include as well.

The idea is to provide some system for users to choose an "icon pack", giving them the choice of which icons to use for their projects.