Set default focus on password text box (#644)

Co-authored-by: AleksandrLiakhavetsEPAM <97155822+AleksandrLiakhavetsEPAM@users.noreply.github.com>

src/Views/Askpass.axaml

@@ -39,9 +39,11 @@
         <TextBlock Text="{Binding Description}" TextWrapping="Wrap"/>
       </Border>
       
-      <TextBox Margin="16"
+      <TextBox Name="PassphraseTextBox"
+               Margin="16"
                MinWidth="300" 
                Height="32" 
+               Focusable="True"
                Text="{Binding Passphrase, Mode=TwoWay}" 
                PasswordChar="*"
                RevealPassword="{Binding ShowPassword, Mode=OneWay}"

src/Views/Askpass.axaml.cs

@@ -52,5 +52,11 @@ namespace SourceGit.Views
             Console.Out.Write($"{Passphrase}\n");
             App.Quit(0);
         }
+
+        protected override void OnOpened(EventArgs e)
+        {
+            PassphraseTextBox.Focus();
+            base.OnOpened(e);
+        }
     }
 }