Onelab "changed" flag is now multi-level (not just true/false). This allows finer granularity for complex clients. For example, Gmsh now responds to 4 levels of "changed"

- changed == 0: nothing to do
- changed == 1: only save the mesh (useful if we only chanaged some physical groups)
- changed == 2: only remesh and save the mesh (useful if we only changed some meshing paramaters)
- changed >= 3: reload geometry, remesh and save the mesh

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- removed onelab::region and onelab::function (they complexify the interface, but bring no additional value compared to onelab::string)
- removed ONELAB2 experiment
- made client/Model name persistent, and define Gmsh/Model name so that we can uncouple e.g. .geo/.pro pairs

After r22501 and r22512 GmshFinalize throws an exception or crashes on most of our test builds; removing for now