removing Marble traces from the Subsurface tree

[Lubomir I. Ivanov]

with the new map widget we started phasing out the Marble globe...
the transition is smooth in the lines of using NO_MARBLE as toggle -
i.e. it either uses Marble or QtLocation.

we should probably start removing it as a dependency from both c++,
cmake and build scripts (i might needs some help for the later). but
the question is - should we? do we want to keep it?

i got no issue reports about the new Map widget which means all is good with it.
so, if we want to remove Marble i can start doing the above within a
new PR with 2-3 patches.

lubomir
--