Minor lobby layout improvements.

Still not as wanted but there are some more issues which need to be
fixed first:

- listbox shouldn't need to call resize after adding items (this is
  probably also the cause of the rather slow display updates of the
  lobby.)

- when listboxes have the vertical grow flag they don't work as
  expected.

Once these items are fixed more lobby improvements will follow.

data/gui/default/window/lobby_main.cfg

@@ -496,7 +496,9 @@
 										[column]
 											horizontal_grow = "true"
 											vertical_alignment = "top"
-											{GAMELISTBOX}
+											{GUI_FORCE_WIDGET_MINIMUM_SIZE 0 "((screen_height * 30) / 100)"  (
+												{GAMELISTBOX}
+											)}
 										[/column]
 									[/row]
 									[row]
@@ -509,7 +511,10 @@
 									[row]
 										[column]
 											horizontal_grow = "true"
-											{_GUI_CHAT_AREA}
+											vertical_grow = "true"
+											{GUI_FORCE_WIDGET_MINIMUM_SIZE 0 "((screen_height * 30) / 100)"  (
+												{_GUI_CHAT_AREA}
+											)}
 										[/column]
 									[/row]
 								[/grid]