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/App.xaml

@@ -15,6 +15,7 @@
                 <ResourceDictionary Source="ms-appx:///CommunityToolkit.WinUI.Controls.Segmented/Segmented/Segmented.xaml" />
                 <ResourceDictionary Source="Styles/ItemContainerStyles.xaml" />
                 <ResourceDictionary Source="Styles/ImagePreviewControl.xaml" />
+                <ResourceDictionary Source="Styles/WebViewEditorControl.xaml" />
 
                 <styles:WinoExpanderStyle />