Fix MessageBox Button Icons

[stepwise-alan]

Pull request type

Please check the type of change your PR introduces:

• Update
• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes

What is the current behavior?

Currently, setting the icon for MessageBox buttons using PrimaryButtonIcon, SecondaryButtonIcon, or CloseButtonIcon properties has no effect. The icons do not appear on the buttons.

Issue Number: N/A

What is the new behavior?

• The PrimaryButtonIcon, SecondaryButtonIcon, and CloseButtonIcon properties now use IconElement instead of SymbolRegular.
• Icons set using these properties are correctly displayed on the MessageBox buttons.

Other information

Before the change:

var uiMessageBox = new Wpf.Ui.Controls.MessageBox
{
    Title = "WPF UI Message Box",
    Content = "Never gonna give you up, never gonna let you down Never gonna run around and desert you Never gonna make you cry, never gonna say goodbye",
    CloseButtonIcon = SymbolRegular.Dismiss24
};
// The icon does not appear on the button.

After the change:

var uiMessageBox = new Wpf.Ui.Controls.MessageBox
{
    Title = "WPF UI Message Box",
    Content = "Never gonna give you up, never gonna let you down Never gonna run around and desert you Never gonna make you cry, never gonna say goodbye",
    CloseButtonIcon = new SymbolIcon(SymbolRegular.Dismiss24)
};
// The icon correctly appears on the button.

[pomianowski]

Hey @stepwise-alan, thanks for the work you've put in