Introducing GeoSceneAbstractTileProjection for tile x/y <-> lonlat

Summary:
In the current code in some places hard assumptions are made about
the projection used by the tile material. Also are calculations
duplicated.
The new abstract class GeoSceneAbstractTileProjection and its currently
two concrete subclasses GeoSceneEquirectTileProjection & GeoSceneMercatorTileProjection
should allow to make most code agnostic of the tile projection details
and also remove the code duplication, also makes unit testing of the projection easy.

GeoSceneAbstractTileProjection follows the concepts of AbstractProjection
and has conversion methods in the interface, with the output vars as
non-const references at the end of the argument list (for consistency, I personally
prefer yielded stuff as return parameter, or have output vars at least being
first in the argument list and as pointers for improved markup in the calling code).
As the current two implementations need to know about the variables
levelZeroColumns and levelZeroRows, I made these properties of the classes
themselves, to avoid having the classes depend on GeoSceneTileDataset.
Comes at the cost of data duplication and thus more complicated setup,
but also avoids an indirection in the class methods using these values.
For simplicity I put these properties onto GeoSceneAbstractTileProjection
itself, to avoid another intermediate abstract subclass for the concept of
levelZeroColumns and levelZeroRows.

The projection methods do not do any out-of-bounds handling, but expect
the calling code to pass proper value. Which should be valid with the current
callees.

Any methods tileProjection() have been renamed to tileProjectionType(),
to make clear those just return the type, not a projection object.

The patch also makes TileId more dump again and just a property container,
especially no longer knowing about GeoSceneTileDataset.

FUTURE WORK:

• Look into all remaining places which have "if tileProjectionType() == x" to see how they can be made tile projection type agnostic as well

Reviewers: rahn, Marble

Reviewed By: rahn, Marble

Subscribers: rahn

Differential Revision: https://phabricator.kde.org/D2780