mouse: set max-content-height on scrolledwindows

Instead of using only hardcoded height requests, it's better
if we give more flexibility for the content to grow up to
a certain amount of pixels.

Also, this patch slightly reduces the maximum size by 20px, so
that in the tested scenario (Adwaita with Large Font settings on)
the panel is still able to size down.

https://bugzilla.gnome.org/show_bug.cgi?id=768529

panels/mouse/gnome-mouse-properties.ui

@@ -21,7 +21,7 @@
     <property name="can_focus">True</property>
     <property name="shadow_type">none</property>
     <property name="hscrollbar_policy">never</property>
-    <property name="height_request">460</property>
+    <property name="max_content_height">440</property>
     <child>
       <object class="GtkBox" id="prefs_widget">
         <property name="visible">True</property>

panels/mouse/gnome-mouse-test.ui

@@ -41,6 +41,7 @@
             <property name="hscrollbar_policy">never</property>
             <property name="vscrollbar_policy">always</property>
             <property name="shadow_type">in</property>
+            <property name="max_content_height">440</property>
             <child>
               <object class="GtkViewport" id="viewport">
                 <property name="visible">True</property>