Merge pull request #2084 from ahopper/add-pointerover-to-ButtonSpinner-and-NumericUpDown-templates

:pointerover effect added to ButtonSpinner

src/Avalonia.Themes.Default/ButtonSpinner.xaml

@@ -87,6 +87,9 @@
           </Grid>
         </Border>
       </ControlTemplate>
-    </Setter>
+    </Setter>    
+  </Style>
+  <Style Selector="ButtonSpinner:pointerover">
+    <Setter Property="BorderBrush" Value="{DynamicResource ThemeBorderMidBrush}"/>
   </Style>
-</Styles>
+</Styles>