tpreferences: used fixed_height = true to enforce side panel icon sizes

For some reason, the 72 px height of the space wasn't functioning as expected.
This does the same job correctly

data/gui/default/window/preferences.cfg

@@ -107,11 +107,12 @@
 		horizontal_placement = "center"
 
 		maximum_width = 900
-		maximum_height = 590
+		maximum_height = 600
 
 		[linked_group]
 			id = "page_icon"
 			fixed_width = "true"
+			fixed_height = "true"
 		[/linked_group]
 
 		[linked_group]
@@ -221,6 +222,7 @@
 														linked_group = "page_icon"
 													[/image]
 												[/column]
+
 												[column]
 													grow_factor = 1
 													horizontal_grow = "true"
@@ -232,14 +234,10 @@
 														linked_group = "page_label"
 													[/label]
 												[/column]
+
 												[column]
-													#
-													# Some icons aren't actually 72px tall, so enforce some
-													# consistency with an invisible spacer.
-													#
 													[spacer]
 														width = 20
-														height = 72
 													[/spacer]
 												[/column]
 											[/row]