LibGfx: Rename Color::from_rgba() => Color::from_argb()

This matches the rename of RGBA32 to ARGB32. It also makes more sense
when you see it used with 32-bit hexadecimal literals:

Before:
    Color::from_rgba(0xaarrggbb)

After:
    Color::from_argb(0xaarrggbb)

Userland/Applets/Audio/main.cpp

@@ -169,7 +169,7 @@ private:
     {
         GUI::Painter painter(*this);
         painter.add_clip_rect(event.rect());
-        painter.clear_rect(event.rect(), Color::from_rgba(0));
+        painter.clear_rect(event.rect(), Color::from_argb(0));
 
         auto& audio_bitmap = choose_bitmap_from_volume();
         painter.blit({}, audio_bitmap, audio_bitmap.rect());