gui2/panel: Avoid using images and just draw rectangles

The opacity used here for the fill colour is identical to the one used
in the image file.

data/gui/widget/panel_campaign_menu.cfg

@@ -32,6 +32,8 @@
 
 					border_thickness = 3
 					border_color = "16, 22, 35, 255"
+
+					fill_color = {GUI__BACKGROUND_COLOR_ENABLED ALPHA=138}
 				[/rectangle]
 
 				[rectangle]
@@ -44,14 +46,6 @@
 					border_color = {GUI__BORDER_COLOR_DARK}
 				[/rectangle]
 
-				[image]
-					x = 3
-					y = 3
-					w = "(width - 6)"
-					h = "(height - 6)"
-					name = "dialogs/translucent54-background.png"
-				[/image]
-
 			[/draw]
 
 		[/background]