Render status property and notification

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are created in various ways, e.g. using old (outdated) data, higher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
Spherical interpolation methods slerp and squad

Adds a method squad() to GeoDataCoordinates for spherical spline
interpolation. Based on the work of Ken Shoemake, implemented according
to Erik B. Dam, Martin Koch, Martin Lillholm "Quaternions,
Interpolation and Animation", page 51,
http://web.mit.edu/2.998/www/QuaternionReport1.pdf

squad will become useful to animate the camera path in KML Tour
playback.

REVIEW: 118751
Squad interpolation example. Shows interpolation of random cities.

Adds an example in examples/cpp/squad-interpolation which picks nearby
cities from Marble's internal city database at random and interpolates
paths between them.

See https://www.youtube.com/watch?v=uieBuzhZjWo

REVIEW: 118751
Smooth playback is awesome
Add a thumbnailer for the supported geo data files (GPX, OSM, KML, ESRI)

FEATURE: Geo Data files (GPX, OSM, KML, ESRI) now can be previewed in the file manager or file dialogs

REVIEW: 118681

Thanks tackat and earthwings for review
Refactored playback logic
Added squad interpolation during playback of tours
Added a progress bar with seek functionality in tour playbacks
Made a more minimalistic user interface with just one Play/Pause togglable button

REVIEW: 118755
SVN_SILENT made messages (.desktop file)
Added "Properties" option on polygons. It includes modifying color
for both lines/polygon interior as well as lines width.
REVIEW: 118766
SVN_SILENT made messages (.desktop file)
tiling
debug++
use www.geonames.org rather than the deprecated ws.geonames.org server

* seems to deliver more results
add search runner