In `Avalonia.Dialogs`, added `readonly` to fields that are never modi… (#13022)

* In `Avalonia.Dialogs`, added `readonly` to fields that are never modified

* Re-added namespace references

* Removed `readonly` modifier from field which is used as a `ref` value

* Removed `readonly` from yet another field

---------

Co-authored-by: Lehonti Ramos <lehonti@ramos>

src/Avalonia.Dialogs/Internal/ManagedFileChooserItemViewModel.cs

@@ -6,7 +6,7 @@ namespace Avalonia.Dialogs.Internal
     {
         private string _displayName;
         private string _path;
-         private DateTime _modified;
+        private DateTime _modified;
         private string _type;
         private long _size;
         private ManagedFileChooserItemType _itemType;

src/Avalonia.Dialogs/Internal/ManagedFileChooserSources.cs

@@ -5,6 +5,7 @@ using System.Linq;
 using Avalonia.Controls.Platform;
 using Avalonia.Utilities;
 
+
 namespace Avalonia.Dialogs.Internal
 {
     internal class ManagedFileChooserSources
@@ -26,7 +27,7 @@ namespace Avalonia.Dialogs.Internal
             return sources.GetUserDirectories().Concat(sources.GetFileSystemRoots()).ToArray();
         }
 
-        private static Environment.SpecialFolder[] s_folders = new[]
+        private static readonly Environment.SpecialFolder[] s_folders = new[]
         {
             Environment.SpecialFolder.Desktop,
             Environment.SpecialFolder.UserProfile,

src/Avalonia.Dialogs/Internal/ManagedFileChooserViewModel.cs

@@ -35,12 +35,12 @@ namespace Avalonia.Dialogs.Internal
         string _fileName;
         private bool _showHiddenFiles;
         private ManagedFileChooserFilterViewModel _selectedFilter;
-        private bool _selectingDirectory;
-        private bool _savingFile;
+        private readonly bool _selectingDirectory;
+        private readonly bool _savingFile;
         private bool _scheduledSelectionValidation;
         private bool _alreadyCancelled = false;
         private string _defaultExtension;
-        private bool _overwritePrompt;
+        private readonly bool _overwritePrompt;
         private CompositeDisposable _disposables;
 
         public string Location