[applets/mediacontroller] Be consistent about which icon to show
Right now the applet shows a play icon when media is currently playing, and a pause icon when it's paused. This makes sense in the abstract, but clashes with the way these icons are used in every other context: to indicate not the current statew, but rather the action to be taken if you click on it. This can even cause an internal inconsistency whereby when the applet is open, its panel representation shows a play icon, but the button in the popup shows a pause icon!
Ultimately I think that while logical and well-intentioned, this winds up being more confusing than tha alternative would be. So let's try following the general standard for how these icons are used.
See the following video with three play/pause icons in three different contexts all changing in sync with one another:
simplescreenrecorder-2021-02-24_12.17.32
BUG: 430592 FIXED-IN: 5.22