One restart for a pack plugin's shader

• Improved: when Nexo, ItemsAdder or another pack plugin builds its pack after MC GUI has started, MC GUI now notices within a minute, merges the new text shader into its own pack and sends that pack again to players online. A shader change used to need two restarts. Now one is enough, and the pack plugin's own reload command should be enough on its own. With resource-pack.mode: external it is still merged on the next restart.