FileIO -> MeshLib::IO.

In order to make OGS more modular:
- The {read/write}Mesh{From/To}File.{h,cpp} are moved from FileIO to MeshLib/IO.
- The FileIO/Legacy folder was moved to MeshLib/IO/Legacy.
- Include paths are changed according the changes above.

When the current commit is also a tag then the version is the tag name,
e.g. "6.0.1". When the current commit is not a tag then the ogs version
is the last tag + number of commits since then + current commit short
hash, e.g. "6.0.1-140-g80ac475".

This info is available in BaseLib::BuildInfo::git_describe. Use it to
uniquely set the OGS version.