[GREENFOOT-841] Stride abstract methods don't load/save params correctly

Stride's loading and saving code for MethodProtoElement doesn't align. The loading code expects the parameters to be children of a "params" sub-element, whereas the saving code saves them as children of the method. This means when you reload a Stride abstract method with parameters, the parameters disappear.

Also, while fixing: it's impossible to add a new parameter to an abstract method using only the keyboard. Pressing right at the end of the name does not go into the empty parameters like it should (like it does for non-abstract methods).

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Issue metadata

• Issue type: Bug
• Priority: Medium
• Fix versions: 3.7.0