Search box visual tweaks (#11105)

Made some changes to the search box:
* Adjusted spacing inside the box
* Detached the search box from the titlebar (as explained [here](#1375 (comment)))
* The search box is now 8px further to the left, in case the scrollbar is always enabled
* Made some controls use default properties, so that they'll adjust nicely to the 2.6 styles

Other: the search box and command palette now use OverlayCornerRadius

Before/After:
![image](https://user-images.githubusercontent.com/84711285/131888377-513b9de4-a653-4086-9a67-8718c64dc75b.png)

src/cascadia/TerminalApp/CommandPalette.xaml

@@ -407,7 +407,7 @@
               HorizontalAlignment="Stretch"
               VerticalAlignment="Top"
               Windows10version1903:Shadow="{StaticResource CommandPaletteShadow}"
-              CornerRadius="{ThemeResource ControlCornerRadius}"
+              CornerRadius="{ThemeResource OverlayCornerRadius}"
               PointerPressed="_backdropPointerPressed"
               Style="{ThemeResource CommandPaletteBackground}">
 

src/cascadia/TerminalControl/SearchBoxControl.xaml

@@ -16,18 +16,23 @@
         <ResourceDictionary>
             <Style x:Key="ToggleButtonStyle"
                    TargetType="ToggleButton">
-                <Setter Property="Width" Value="25" />
-                <Setter Property="Height" Value="25" />
+                <Setter Property="Width" Value="24" />
+                <Setter Property="Height" Value="24" />
                 <Setter Property="Background" Value="Transparent" />
-                <Setter Property="Padding" Value="1" />
-                <Setter Property="CornerRadius" Value="2" />
+                <Setter Property="BorderBrush" Value="Transparent" />
+                <Setter Property="Padding" Value="0" />
+                <Setter Property="Margin" Value="2,0" />
+                <Setter Property="CornerRadius" Value="{ThemeResource ControlCornerRadius}" />
             </Style>
             <Style x:Key="ButtonStyle"
                    TargetType="Button">
-                <Setter Property="Width" Value="25" />
-                <Setter Property="Height" Value="25" />
+                <Setter Property="Width" Value="24" />
+                <Setter Property="Height" Value="24" />
                 <Setter Property="Background" Value="Transparent" />
-                <Setter Property="CornerRadius" Value="2" />
+                <Setter Property="BorderBrush" Value="Transparent" />
+                <Setter Property="Padding" Value="0" />
+                <Setter Property="Margin" Value="6,0,0,0" />
+                <Setter Property="CornerRadius" Value="{ThemeResource ControlCornerRadius}" />
             </Style>
             <ResourceDictionary.ThemeDictionaries>
                 <ResourceDictionary x:Key="Dark">
@@ -227,14 +232,15 @@
         </ResourceDictionary>
     </UserControl.Resources>
 
-    <StackPanel Padding="5"
-                CornerRadius="0,0,2,2"
+    <StackPanel Margin="8"
+                Padding="8"
+                CornerRadius="{ThemeResource OverlayCornerRadius}"
                 Orientation="Horizontal"
                 Style="{ThemeResource SearchBoxBackground}">
         <TextBox x:Name="TextBox"
                  x:Uid="SearchBox_TextBox"
                  Width="160"
-                 Margin="5"
+                 Margin="0,0,6,0"
                  HorizontalAlignment="Left"
                  VerticalAlignment="Center"
                  CornerRadius="2"
@@ -270,7 +276,6 @@
 
         <Button x:Name="CloseButton"
                 x:Uid="SearchBox_Close"
-                Padding="0"
                 Click="CloseClick"
                 Style="{ThemeResource ButtonStyle}">
             <FontIcon FontFamily="Segoe MDL2 Assets"