Fix handling of full-length hex values with '#' symbol in ColorPicker

This allows pasting i.e. '#FFFFFFFF' in the hex input text box

src/Avalonia.Controls.ColorPicker/Themes/Fluent/ColorPicker.xaml

@@ -245,11 +245,12 @@
                                        VerticalAlignment="Center" />
                           </Border>
                           <!-- Color updated in code-behind -->
+                          <!-- Max length must include an optional '#' prefix (#FFFFFFFF) -->
                           <TextBox x:Name="PART_HexTextBox"
                                    Grid.Column="1"
                                    AutomationProperties.Name="Hexadecimal Color"
                                    Height="32"
-                                   MaxLength="8"
+                                   MaxLength="9"
                                    HorizontalAlignment="Stretch"
                                    CornerRadius="0,4,4,0" />
                         </Grid>

src/Avalonia.Controls.ColorPicker/Themes/Fluent/ColorView.xaml

@@ -494,11 +494,12 @@
                                  VerticalAlignment="Center" />
                     </Border>
                     <!-- Color updated in code-behind -->
+                    <!-- Max length must include an optional '#' prefix (#FFFFFFFF) -->
                     <TextBox x:Name="PART_HexTextBox"
                              Grid.Column="1"
                              AutomationProperties.Name="Hexadecimal Color"
                              Height="32"
-                             MaxLength="8"
+                             MaxLength="9"
                              HorizontalAlignment="Stretch"
                              CornerRadius="0,4,4,0" />
                   </Grid>

src/Avalonia.Controls.ColorPicker/Themes/Simple/ColorPicker.xaml

@@ -245,11 +245,12 @@
                                        VerticalAlignment="Center" />
                           </Border>
                           <!-- Color updated in code-behind -->
+                          <!-- Max length must include an optional '#' prefix (#FFFFFFFF) -->
                           <TextBox x:Name="PART_HexTextBox"
                                    Grid.Column="1"
                                    AutomationProperties.Name="Hexadecimal Color"
                                    Height="32"
-                                   MaxLength="8"
+                                   MaxLength="9"
                                    HorizontalAlignment="Stretch"
                                    CornerRadius="0,0,0,0" />
                         </Grid>

src/Avalonia.Controls.ColorPicker/Themes/Simple/ColorView.xaml

@@ -456,11 +456,12 @@
                                  VerticalAlignment="Center" />
                     </Border>
                     <!-- Color updated in code-behind -->
+                    <!-- Max length must include an optional '#' prefix (#FFFFFFFF) -->
                     <TextBox x:Name="PART_HexTextBox"
                              Grid.Column="1"
                              AutomationProperties.Name="Hexadecimal Color"
                              Height="32"
-                             MaxLength="8"
+                             MaxLength="9"
                              HorizontalAlignment="Stretch"
                              CornerRadius="0,0,0,0" />
                   </Grid>