Make content assignment Qt 5.2+ (?) compatible

For some reason, on Jolla it is possible to do this in BasePAge instances:

content {
    Item {}
}

But it is an error when running with Qt 5.3 and I'm almost certain that
also with 5.2. As we want the modRana GUI to be robust and multi-platform,
we need to use the method that works on both Jolla and normal Qt, even if it
might be a bit less convenient in some places.
Replace anchors with column on the About page

This also results in correct content field assignment.
Use compatible content field assignments
Reorganize Qt 5 GUI QML files

Move reusable elements to the modrana_components subfolder and non-graphical
backend/logic components to the backend folder.

With this there should be only individual page instances and the
main.qml (rWin) file in the toplevel folder.