Allow to modify parameters and replaceable classes of the model itself in OMEdit

[casella]

Description

When visualizing a model's diagram, OMEdit allows to modify the parameter values of models instantiated within it, by double-clicking on their icons and showing parameter input masks. However, it does not allow to change the default values of the parameters of the model itself, which will also come in handy in many cases.

Steps to Reproduce

Open the following model

model ModifyParameters
  extends Modelica.Blocks.Interfaces.SO;
  parameter Real p = 10;
equation
  y = 2*p;
end ModifyParameters;

in OMEdit. The only way to change the value of p is to switch to the textual view and edit the code manually.

Expected Behavior

It would be good if one could right-click on the icon or diagram layers, select a menu item "Parameters" and then get the same kind of input window that one gets to change modifiers in instantiated components.

@adeas31, would you need a new API function to handle this?

[casella]

Probably also related to the structural changes needed to solve #2081