Data needed for rendering (texture tiles, search results, routes, ...)
is often download from the Internet and Marble renders preliminary
results while waiting for the data to arrive. The preliminary results
are created in various ways, e.g. using old (outdated) data, higher
level tiles or partial data. We inform the user to some extend about
pending data (e.g. progress bar for tile downloads, busy indicator for
routes), but there is no sane programmatical way at the moment to
determine whether all data has arrived yet.

The patch adds an enum property RenderStatus { Complete,
WaitingForUpdate, WaitingForData, Incomplete } to MarbleMap and
MarbleWidget along with a renderStatusChanged() notification signal.
LayerManager maintains that status value. Internally it delegates the
status determination to all layers and uses a tree-like data structure
to record all details.

REVIEW: 118618