Extract webvieweditor to share between compose page and signature editor (#578)

* initial work for webview editor control

* moved more stuff to editor itself

* revert packages.props indention changes

* move alignment logic

* Migrate signature editor to new control

* move background to editor control

* Some polishing

* Fixed the corner glitch issue with dark theme.

---------

Co-authored-by: Burak Kaan Köse <bkaankose@outlook.com>

Wino.Mail/Styles/WebViewEditorControl.xaml

@@ -0,0 +1,21 @@
+<ResourceDictionary
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:controls="using:Wino.Mail.Controls"
+    xmlns:muxc="using:Microsoft.UI.Xaml.Controls">
+
+    <Style TargetType="controls:WebViewEditorControl">
+        <Style.Setters>
+            <Setter Property="Template">
+                <Setter.Value>
+                    <ControlTemplate TargetType="controls:WebViewEditorControl">
+                        <Grid CornerRadius="3">
+                            <Grid Background="White" Visibility="{Binding RelativeSource={RelativeSource Mode=TemplatedParent}, Path=IsEditorDarkMode, Mode=OneWay, Converter={StaticResource ReverseBooleanToVisibilityConverter}}" />
+                            <muxc:WebView2 x:Name="WebView" />
+                        </Grid>
+                    </ControlTemplate>
+                </Setter.Value>
+            </Setter>
+        </Style.Setters>
+    </Style>
+</ResourceDictionary>