ensure ListBoxes always have enough height (#2084)

so they can always display an error message if the filter doesn't match anything

docs/changelog.txt

@@ -70,6 +70,7 @@ changelog.txt uses a syntax similar to RST, with a few special sequences:
 - Added functions reverse-engineered from ambushing unit code: ``Units::isHidden``, ``Units::isFortControlled``, ``Units::getOuterContainerRef``, ``Items::getOuterContainerRef``
 
 ## Lua
+- ``widgets.ListBox``: minimum height of dialog is now calculated correctly when there are no items in the list (e.g. when a filter doesn't match anything)
 - Lua wrappers for functions reverse-engineered from ambushing unit code: ``isHidden(unit)``, ``isFortControlled(unit)``, ``getOuterContainerRef(unit)``, ``getOuterContainerRef(item)``
 - ``dwarfmode.enterSidebarMode()``: passing ``df.ui_sidebar_mode.DesignateMine`` now always results in you entering ``DesignateMine`` mode and not ``DesignateChopTrees``, even when you looking at the surface where the default designation mode is ``DesignateChopTrees``
 - New string class function: ``string:escape_pattern()`` escapes regex special characters within a string

library/lua/gui/dialogs.lua

@@ -212,10 +212,10 @@ function ListBox:onRenderFrame(dc,rect)
 end
 
 function ListBox:getWantedFrameSize()
-    local mw, mh = InputBox.super.getWantedFrameSize(self)
+    local mw, mh = ListBox.super.getWantedFrameSize(self)
     local list = self.subviews.list
     list.frame.t = mh+1
-    return math.max(mw, list:getContentWidth()), mh+1+math.min(20,list:getContentHeight())
+    return math.max(mw, list:getContentWidth()), mh+3+math.min(18,list:getContentHeight())
 end
 
 function ListBox:onInput(keys)