Add User Highlighted icon for plugins

[pStyl3]

In the loot-discussions channel on Discord, Skinjack requested that we add the ability to mark plugins with an icon, simply for the purpose of reminding themselves that they need to check or reevaluate the plugin later once more.

[Ortham]

LOOT would need to store a list of user-highlighted plugins somewhere: since that's game-specific state, we could have a highlighted.txt file in each game directory that just just a list of plugins (one per line) that the user has highlighted, and LOOT could load this file on start and write it whenever the user highlights or unhighlights a plugin.