This way, we can cover string and QIcon assignments transparently,
another backwards compatibility fix, and easier thing in Plasma2: Now
the user doesn't have to worry about creating QIcons, but can just
assign the icon name, and it will get loaded from the theme. Models that
provide QIcons (such as QStandardItems) now work correctly with
PlasmaCore.MenuItems.
In MenuItem, we can't inherit QAction anymore (our MenuItem has to be a
QQuickItem), so we need to aggregate, which means adding glue API.
Positioning code is disabled, icons broken, but a QMenu with our items
inside shows up and is works.
All cpp code moves into the src/ subdirectory, as the Frameworks policy
suggests.
Directory structure should now be in line with other, future frameworks.
