pages/powerfulwhenneeded: Split layer into separate files & reorder

• qml/Main: Change id to access throughout app for createPage
  • And rework page functions to make clear what distro pages should be able to do.
• pages/PowerfulWhenNeeded: Split layers into separate files & center buttons
  • This reduces the complexity of the page and makes it easier to extend the layer pages. If a layer page can't be loaded dynamically, it can now also show an error page, but the main page and other layers can still continue to work.
• pages/PowerfulWhenNeeded: Reorder feature buttons
  • Present more popular features first, and more obscure ones later. System Settings is the final item, as it book-ends the set nicely.

Before	After