Don't expose pseudoclass constants.

1. They don't follow naming conventions
2. If we're going to expose them, why just `protected`?
3. If we do it, we should do it on all controls

src/Avalonia.Controls/Button.cs

@@ -34,8 +34,8 @@ namespace Avalonia.Controls
     [PseudoClasses(pcFlyoutOpen, pcPressed)]
     public class Button : ContentControl, ICommandSource, IClickableControl
     {
-        protected const string pcPressed    = ":pressed";
+        private const string pcPressed    = ":pressed";
-        protected const string pcFlyoutOpen = ":flyout-open";
+        private const string pcFlyoutOpen = ":flyout-open";
 
         /// <summary>
         /// Defines the <see cref="ClickMode"/> property.

src/Avalonia.Controls/CalendarDatePicker/CalendarDatePicker.cs

@@ -26,8 +26,8 @@ namespace Avalonia.Controls
     [PseudoClasses(pcFlyoutOpen, pcPressed)]
     public partial class CalendarDatePicker : TemplatedControl
     {
-        protected const string pcPressed    = ":pressed";
+        private const string pcPressed    = ":pressed";
-        protected const string pcFlyoutOpen = ":flyout-open";
+        private const string pcFlyoutOpen = ":flyout-open";
 
         private const string ElementTextBox = "PART_TextBox";
         private const string ElementButton = "PART_Button";

src/Avalonia.Controls/ComboBox.cs

@@ -21,8 +21,9 @@ namespace Avalonia.Controls
     [PseudoClasses(pcDropdownOpen, pcPressed)]
     public class ComboBox : SelectingItemsControl
     {
-        public const string pcDropdownOpen = ":dropdownopen";
+        internal const string pcDropdownOpen = ":dropdownopen";
-        public const string pcPressed = ":pressed";
+        internal const string pcPressed = ":pressed";
+
         /// <summary>
         /// The default value for the <see cref="ItemsControl.ItemsPanel"/> property.
         /// </summary>

src/Avalonia.Controls/SplitButton/SplitButton.cs

@@ -18,9 +18,9 @@ namespace Avalonia.Controls
     [PseudoClasses(pcFlyoutOpen, pcPressed)]
     public class SplitButton : ContentControl, ICommandSource
     {
-        protected const string pcChecked    = ":checked";
+        internal const string pcChecked    = ":checked";
-        protected const string pcPressed    = ":pressed";
+        internal const string pcPressed    = ":pressed";
-        protected const string pcFlyoutOpen = ":flyout-open";
+        internal const string pcFlyoutOpen = ":flyout-open";
 
         /// <summary>
         /// Raised when the user presses the primary part of the <see cref="SplitButton"/>.

src/Avalonia.Controls/SplitView/SplitView.cs

@@ -22,15 +22,15 @@ namespace Avalonia.Controls
     [PseudoClasses(pcLightDismiss)]
     public class SplitView : ContentControl
     {
-        protected const string pcOpen = ":open";
+        private const string pcOpen = ":open";
-        protected const string pcClosed = ":closed";
+        private const string pcClosed = ":closed";
-        protected const string pcCompactOverlay = ":compactoverlay";
+        private const string pcCompactOverlay = ":compactoverlay";
-        protected const string pcCompactInline = ":compactinline";
+        private const string pcCompactInline = ":compactinline";
-        protected const string pcOverlay = ":overlay";
+        private const string pcOverlay = ":overlay";
-        protected const string pcInline = ":inline";
+        private const string pcInline = ":inline";
-        protected const string pcLeft = ":left";
+        private const string pcLeft = ":left";
-        protected const string pcRight = ":right";
+        private const string pcRight = ":right";
-        protected const string pcLightDismiss = ":lightDismiss";
+        private const string pcLightDismiss = ":lightDismiss";
 
         /// <summary>
         /// Defines the <see cref="CompactPaneLength"/> property