Fix and tidy the Character select, Shortcuts, Log and QR popups

Character select
- A card only picks its character when it is clicked. Before, every card
  watched every click, so clicking below the panel could pick a card
  scrolled out of view there.
- Six columns of fixed-size cards that scroll for any number of
  characters, with a gap between the cards and the scrollbar.
- Pictures are read as PNG, JPG or WebP by their first bytes, so JPGs no
  longer print an error. Names are split into words ("Paper Mario") and
  the card under the mouse is highlighted.

Shortcuts
- Alt+Z restarts the song; it was listed as "Reset".
- Grouped by what the keys do, in sentence case, and the panel fits the
  list. A test checks that every button shortcut is listed.

Log
- Scrolls to the newest row, alternates row colours and says "Nothing
  has happened yet" when empty.

QR
- A title, and the code is set up in one place.

popups/qr_popup.gd

@@ -9,21 +9,18 @@ extends BasePopup
 
 
 func _ready() -> void:
-	qr_code_rect.light_module_color = Color.WHITE
-	qr_code_rect.dark_module_color = Color.BLACK
-	qr_code_rect.quiet_zone_size = 4
-
-	# Generate QR code for the search URL
-	var url: String = Settings.default_path + "/search"
-	qr_code_rect.data = url
-	url_label.text = qr_code_rect.data
-	GameLog.debug("QR code generated for: ", url)
+	_show_search_url()
 
 
 func open() -> void:
 	super()
-	# Regenerate QR code in case settings changed
-	var url: String = Settings.default_path + "/search"
-	qr_code_rect.data = url
-	url_label.text = qr_code_rect.data
+	# Another server may have been picked in the settings since last time
+	_show_search_url()
 
+
+func _show_search_url() -> void:
+	var url: String = Settings.default_path + "/search"
+	if qr_code_rect.data != url:
+		qr_code_rect.data = url
+		GameLog.debug("QR code generated for: ", url)
+	url_label.text = url