Change DashedLine default visibility to collapsed (#1019)

The X2 binding is causing the textbox to never give up any width it's gotten. The visibility change makes it so it's only ever an issue if the textbox is disabled and the text is shortend programmatically.

Author: jizc

MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.TextBox.xaml

@@ -81,7 +81,7 @@
                                                    />
                                 </Grid>
                             </Border>
-                            <Line x:Name="DashedLine" VerticalAlignment="Bottom" Visibility="Hidden"
+                            <Line x:Name="DashedLine" VerticalAlignment="Bottom" Visibility="Collapsed"
                                   StrokeThickness="1.25" StrokeDashArray="1,2.5" StrokeDashCap="Round"
                                   X1="0" X2="{Binding ActualWidth, ElementName=border}" Y1="0" Y2="0" 
                                   Stroke="{TemplateBinding BorderBrush}" Opacity="0.56" />