[CO] Merging DMatLib and DMatSetup + [CO] Loading missing files

[BonneelP]

Hello all,

We are currently working to simplify/improve the material library widgets. As a first (important) step, we have merged DMatLib and DMatSetup into one widget:
Before:

Now:

So instead of opening a popup when editing/creating a new material everything is done directly in DMatLib which should be clearer for the user (since the material list is still visible on the side).

The buttons "Add to machine" and "Save in Library" were a source of confusion. Now when selecting a material in the Library a button "Edit in machine" is available and add a line in the corresponding table (still on the same widget so the consequence of clicking this button should be more clear).
For now the GUI doesn't allow any more to save a material from a machine into the Library. We could add this feature back with a dedicated button but it would add some complexity to the GUI. We think that we should encourage creating first a material in the Library before editing it in the machine.

When saving a machine/simulation in json folder mode, the machine file save the materials as mat_type: "M400-50A.json" as this file is supposed to be next to the machine. When sending a machine by email, it happens that the material are forgotten and it become impossible to open the machine file in the GUI. The class generator was modified to set a missing file to None (a log a error message) to enable the possibility to open the machine without its materials.

This PR also contains some miscellaneous bug correction/improvements:

• Slot constraints in the GUI are cleaner
• try catch were added to the plot in the GUI
• the retro-compatibility now use the dedicated logger

Best Regards,
Pierre