Fix FilledTextField hint margin issue (#1575)

Great work! Thank you.

Author: AmRo045

MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.TextBox.xaml

@@ -191,6 +191,14 @@
                             <Setter TargetName="Hint" Property="Margin" Value="0,0,0,16" />
                             <Setter TargetName="PART_ContentHost" Property="Margin" Value="0,8,0,8" />
                         </Trigger>
+                        <MultiTrigger>
+                            <MultiTrigger.Conditions>
+                                <Condition Property="wpf:TextFieldAssist.HasFilledTextField" Value="True" />
+                                <Condition Property="wpf:HintAssist.IsFloating" Value="False" />
+                            </MultiTrigger.Conditions>
+                            <Setter TargetName="PART_ContentHost" Property="Margin" Value="0,0,0,8" />
+                            <Setter TargetName="Hint" Property="Margin" Value="0,0,0,0" />
+                        </MultiTrigger>
                         <MultiTrigger>
                             <MultiTrigger.Conditions>
                                 <Condition Property="wpf:TextFieldAssist.HasOutlinedTextField" Value="False" />