Facilitating custom pages

[kkjamie]

This PR implements a mechanism so we can easily add new pages to the debug menu.

Projects can now add their own pages with no code in the debug menu itself
It also makes it easier for us to extend the core pages we ship with the debug menu.

Key features:

• Abstract class at the top where you serialize the button name + back button
• Adds a button automatically (optional - just leave the buttonText field blank and it's a secret hidden page, you can open it via code)
• API to access the pages via code: from the debug menu debugMenu.GetPage<PageType>().
• Auto back button logic to return to the root menu of the debug menu.

Known potential issues:

• Naming: AbstractDebugMenuTabPage - it's not really a "Tab", although the UI kinda looks like that.
• Could do with a scroll view incase you add lots and lots of pages or you are on a small screen.

I can address the 2 things above after some PR feedback