(This is just a band-aid to avoid crashes when updating widgets interactively. We should really avoid rebulding the tree all the time)

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

better GetNumber and GetString parser functions (old GetString function has been renamed GetStringValue, to match GetValue)

- 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

Make RunOnelabClient work also when Gmsh is a non-local ONELAB client (i.e., same as a Python subclient)

allow lists in DefineConstant + on command line with -setlistofnumbers + propagate info to onelab clients

- new GmshExecutableName to get name of exe