CharacterMap: Allow resizing the unicode block list

Previously we would constrain the unicode block list to a width of 175,
causing it to stick to the splitter when manually resizing.

This patch allows resizing the list properly while retaining the new
width when resizing the window.

Userland/Applications/CharacterMap/CharacterMapWindow.gml

@@ -30,7 +30,7 @@
     }
 
     @GUI::HorizontalSplitter {
-        fixed_resizee: "Second"
+        opportunistic_resizee: "First"
 
         @GUI::Widget {
             layout: @GUI::VerticalBoxLayout {}
@@ -59,7 +59,7 @@
         }
 
         @GUI::ListView {
-            max_width: 175
+            preferred_width: 175
             name: "unicode_block_listview"
         }
     }