Clicking '+' in Water Chemistry tool terminates the application with an assert

[matty0ung]

Last few log lines:

[18:23:36.485] (17yk386z28) DEBUG : void TableModelBase<Derived, NE>::add(std::shared_ptr<_Tp>) [with Derived = RecipeAdjustmentSaltTableModel; NE = RecipeAdjustmentSalt] ""  [tableModels/TableModelBase.h:289]
[18:23:36.485] (17yk386z28) DEBUG : Recipe* TableModelBase<Derived, NE>::doGetObservedRecipe() const requires (IsTableModel<Caller>) && (ObservesRecipe<Caller>) [with Caller = RecipeAdjustmentSaltTableModel; Derived = RecipeAdjustmentSaltTableModel; NE = RecipeAdjustmentSalt] Substantive version  [tableModels/TableModelBase.h:373]
[18:23:36.485] (17yk386z28) WARNING : Salt* RecipeAdjustmentSalt::salt() const No Salt set on RecipeAdjustmentSalt # -1  [model/RecipeAdjustmentSalt.cpp:97]
[18:23:36.485] (17yk386z28) DEBUG : QVariant PropertyPath::getValue(const NamedEntity&) const Property salt returned nullptr  [utils/PropertyPath.cpp:179]
[18:23:36.485] (17yk386z28) ERROR : QVariant TableModelBase<Derived, NE>::readDataFromModel(const QModelIndex&, int) const [with Derived = RecipeAdjustmentSaltTableModel; NE = RecipeAdjustmentSalt] Unable to read RecipeAdjustmentSalt # -1 property "salt/name" (Got QVariant(Invalid) )  [tableModels/TableModelBase.h:507]
[18:23:36.485] (17yk386z28) ERROR : ASSERT: "false" in file ../src/tableModels/TableModelBase.h, line 510  [tableModels/TableModelBase.h:510]
Abandon (core dumped)

I know where to start looking for this, but it's a slightly different bug from the ones in #817, so giving it its own issue.

Part of the issue is that we have not previously managed salts in quite the same way as other ingredients. Now that we've more closely aligned how recipe additions work, we need to create a bit of extra logic for creating and managing salts in the same way as fermentables, hops, misc items etc. It will then be more straightforward to get salt additions to work. (This is the plan, anyway.)