Updated icon! Also, button's icon won't shrink if you resize your browser's window, just to match with the X button next to it.

QoL:
- Button now shows only if there’s text in the YouTube search box.
- Button's icon color now matches with YouTube's search box's icon (used to be gray but now white).
- Lower CPU usage / better battery life! Removed frequent background polling and navigation detection is event-driven.
- Other minor adjustments.