Fix crash when initializing Nokia place categories.

If network errors occur while initializing categories stale categories
were left in the category tree which can result in unbounded recursion.

Change-Id: I93a3b7a01f152473d25b492bba579b9300639bba
Remove dependency between MapStyle and Nokia map schemes

Makes it possible to add Nokia base schemes without adding new
MapStyles.

Removes the redundant collection of base schemes.

Change-Id: Ia2189a3f10ba47cc40087b7d4fee9a2c5d84caad
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
Reviewed-by: Aaron McCarthy <mccarthy.aaron@gmail.com>
Tile cache versioning for Nokia maps

Added to avoid mixing of different map versions in map view.

Generic cache versioning by adding an incremental version number to
tile specs and filenames. Default version -1 falls back to not using
version in file names for backward compatibility.

Nokia specific version management by implementing the version request,
comparing all version information and increment the version if anything
has changed.

Task-number: QTBUG-25559
Change-Id: I6820f2efbe7458701475cc833d3077022797b2df
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>