Remove WindowDecorationsHelper, fix theme in properties window

[gave92]

Resolved / Related Issues
Items resolved / related issues by this PR.

• For WinUI 3 progress #9780 ("Window Decoration api isn't working on the properties windows (we should remove the relevant code before release)")
• Closes Bug: Title bar close/minimize/maximize buttons color don't match the app theme #9911

Changes

• Removed AppWindowTitleBar.IsCustomizationSupported() check, not necessary as of winappsdk 1.2 (titlebar API supported down to 1809)
• Removed WindowDecorationsHelper, uwp only + same reason as above
• Removed code for custom titlebar close button (it was commented), we now use the standard close button
• Fix wrong background in properties window when theme changes
• Fix crash when clicking "share", remove call to GetForCurrentView() (uwp only)
• Restore code for changing titlebar buttons on theme change
• Improve positioning of properties window (do not go out of screen bounds)

Validation
How did you test these changes?

• Built and ran the app

[yaira2]

Restore code for changing titlebar buttons on theme change

@gave92 this seems to help for the main window, but the issue still exists with the property windows.

[gave92]

Should be good now.

[yaira2]

LGTM