Added resource for TextControlPlaceholderOpacity (#19705)

* Added resource for TextControlPlaceholderOpacity

This allows overriding of the opacity instead of it being hardcoded to 50%.

* Added `TextControlPlaceholderOpacity` to Simple theme TextBox

src/Avalonia.Themes.Fluent/Accents/FluentControlResources.xaml

@@ -240,6 +240,7 @@
       <StaticResource x:Key="TextControlBackgroundDisabled" ResourceKey="SystemControlBackgroundBaseLowBrush" />
       <StaticResource x:Key="TextControlBorderBrushFocused" ResourceKey="SystemControlHighlightAccentBrush" />
       <StaticResource x:Key="TextControlBorderBrushDisabled" ResourceKey="SystemControlDisabledBaseLowBrush" />
+      <x:Double x:Key="TextControlPlaceholderOpacity">0.5</x:Double>
       <StaticResource x:Key="TextControlPlaceholderForeground" ResourceKey="SystemControlPageTextBaseMediumBrush" />
       <StaticResource x:Key="TextControlPlaceholderForegroundPointerOver"
                       ResourceKey="SystemControlPageTextBaseMediumBrush" />

src/Avalonia.Themes.Fluent/Controls/TextBox.xaml

@@ -145,7 +145,7 @@
                                 BringIntoViewOnFocusChange="{TemplateBinding (ScrollViewer.BringIntoViewOnFocusChange)}">
                     <Panel>
                       <TextBlock Name="PART_Watermark"
-                              Opacity="0.5"
+                              Opacity="{DynamicResource TextControlPlaceholderOpacity}"
                               Text="{TemplateBinding Watermark}"
                               TextAlignment="{TemplateBinding TextAlignment}"
                               TextWrapping="{TemplateBinding TextWrapping}"

src/Avalonia.Themes.Simple/Accents/Base.xaml

@@ -151,4 +151,6 @@
 
   <sys:Double x:Key="IconElementThemeHeight">20</sys:Double>
   <sys:Double x:Key="IconElementThemeWidth">20</sys:Double>
+
+  <sys:Double x:Key="TextControlPlaceholderOpacity">0.5</sys:Double>
 </ResourceDictionary>

src/Avalonia.Themes.Simple/Controls/TextBox.xaml

@@ -136,7 +136,7 @@
                     <TextBlock Name="watermark"
                                HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                                VerticalAlignment="{TemplateBinding VerticalContentAlignment}"   
-                               Opacity="0.5"
+                               Opacity="{DynamicResource TextControlPlaceholderOpacity}"
                                Text="{TemplateBinding Watermark}"
                                TextAlignment="{TemplateBinding TextAlignment}"
                                TextWrapping="{TemplateBinding TextWrapping}">